BUG: pd.to_timedelta handles missing data
closes #5437 

**Updated**: Mostly following @jreback's suggestion, but we need `np.timedelta64('NaT')`, not `pd.tslib.iNaT`, because numpy cannot cast `np.datetime64('NaT')` as `np.timedelta64` type, and throws an `AssertionError`.

```
In [1]: pd.to_timedelta(Series(['00:00:01', '00:00:02']))
Out[1]: 
0   00:00:01
1   00:00:02
dtype: timedelta64[ns]

In [2]: pd.to_timedelta(Series(['00:00:01', np.nan]))
Out[2]: 
0   00:00:01
1        NaT
dtype: timedelta64[ns]

In [3]: pd.to_timedelta(Series(['00:00:01', pd.NaT]))
Out[3]: 
0   00:00:01
1        NaT
dtype: timedelta64[ns]
```

Another issue lurking...

```
In [76]: Series([np.timedelta64('NaT')], dtype='<m8[ns]') + pd.to_timedelta('00:00:01')
Out[76]: 
0   NaT
dtype: timedelta64[ns]

In [77]: DataFrame([np.timedelta64('NaT')], dtype='<m8[ns]') + pd.to_timedelta('00:00:01')
Out[77]: 
                              0
0 -106751 days, 23:47:15.854776
```
 fyi....this test should hold as well:

`pd.to_datetime(list_of_strings) == Series(list_of_strings).apply(pd.to_datetime)`
 This patch will fix the problem from above

need a sequence of tests (you can do in a single method if y ou want, just need these combinations).

`x op y`

where op in `+, -`
x and y in `Series(valid_td), Series(NaT), Frame(valid_td), Frame(NaT), NaT, timedelta64 scalar`

```
diff --git a/pandas/core/internals.py b/pandas/core/internals.py
index 62aa95d..209b960 100644
--- a/pandas/core/internals.py
+++ b/pandas/core/internals.py
@@ -1162,15 +1162,25 @@ class TimeDeltaBlock(IntBlock):

     def _try_coerce_args(self, values, other):
         """ provide coercion to our input arguments
-            we are going to compare vs i8, so coerce to integer
-            values is always ndarra like, other may not be """
-        values = values.view('i8')
+            we are going to compare vs i8, so coerce to floats
+            repring NaT with np.nan so nans propogate
+            values is always ndarray like, other may not be """
+        def masker(v):
+            mask = isnull(v)
+            v = v.view('i8').astype('float64')
+            v[mask] = np.nan
+            return v
+
+        values = masker(values)
+
         if isnull(other) or (np.isscalar(other) and other == tslib.iNaT):
-            other = tslib.iNaT
+            other = np.nan
         elif isinstance(other, np.timedelta64):
             other = _coerce_scalar_to_timedelta_type(other,unit='s').item()
+            if other == tslib.iNaT:
+                other = np.nan
         else:
-            other = other.view('i8')
+            other = masker(v)

         return values, other
```
 Take a look at the comments near my tests. There is still one major issue:

```
In [6]: s1
Out[6]:
0   00:00:01
dtype: timedelta64[ns]

In [7]: df1
Out[7]:
         0
0 00:00:01

In [8]: s1 + df1
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-8-48d4d6969771> in <module>()
----> 1 s1 + df1

/Users/danielallan/Documents/Repos/pandas-danielballan/pandas/core/ops.pyc in wrapper(left, right, name)
    453     def wrapper(left, right, name=name):
    454
--> 455         time_converted = _TimeOp.maybe_convert_for_time_op(left, right, name)
    456
    457         if time_converted is None:

/Users/danielallan/Documents/Repos/pandas-danielballan/pandas/core/ops.pyc in maybe_convert_for_time_op(cls, left, right, name)
    422         if name.startswith('__r'):
    423             name = "__" + name[3:]
--> 424         return cls(left, right, name)
    425
    426

/Users/danielallan/Documents/Repos/pandas-danielballan/pandas/core/ops.pyc in __init__(self, left, right, name)
    256
    257         lvalues = self._convert_to_array(left, name=name)
--> 258         rvalues = self._convert_to_array(right, name=name)
    259
    260         self.is_timedelta_lhs = com.is_timedelta64_dtype(left)

/Users/danielallan/Documents/Repos/pandas-danielballan/pandas/core/ops.pyc in _convert_to_array(self, values, name)
    338         elif inferred_type == 'integer':
    339             # py3 compat where dtype is 'm' but is an integer
--> 340             if values.dtype.kind == 'm':
    341                 values = values.astype('timedelta64[ns]')
    342             elif isinstance(values, pd.PeriodIndex):

/Users/danielallan/Documents/Repos/pandas-danielballan/pandas/core/generic.pyc in __getattr__(self, name)
   1504             return self[name]
   1505         raise AttributeError("'%s' object has no attribute '%s'" %
-> 1506                              (type(self).__name__, name))
   1507
   1508     def __setattr__(self, name, value):

AttributeError: 'DataFrame' object has no attribute 'dtype'
```
 I did a PR on your branch...incorporate the last commit..and should be good to go (though may need a couple of comparisons for the fixed tests)
 Thanks.

I may have screwed up your PR. Thought I merged it, now don't see it....
 nm, I see what I did. Thanks again for handling the hard parts of this. Stand by.
 That worked.

I added a test to check

`pd.to_datetime(list_of_strings) == Series(list_of_strings).apply(pd.to_datetime)`

and I think apply is returning objects instead of timedelta64[ns]. Is this another case of taking the wrong path?

```
FAIL: test_apply_to_timedelta (pandas.tseries.tests.test_timedeltas.TestTimedeltas)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/danielallan/Documents/Repos/pandas-danielballan/pandas/tseries/tests/test_timedeltas.py", line 302, in test_apply_to_datetime
    assert_series_equal(a, b)
  File "/Users/danielallan/Documents/Repos/pandas-danielballan/pandas/util/testing.py", line 417, in assert_series_equal
    assert_attr_equal('dtype', left, right)
  File "/Users/danielallan/Documents/Repos/pandas-danielballan/pandas/util/testing.py", line 401, in assert_attr_equal
    assert_equal(left_attr,right_attr,"attr is not equal [{0}]" .format(attr))
  File "/Users/danielallan/Documents/Repos/pandas-danielballan/pandas/util/testing.py", line 384, in assert_equal
    assert a == b, "%s: %r != %r" % (msg.format(a,b), a, b)
AssertionError: attr is not equal [dtype]: dtype('<m8[ns]') != dtype('O')
```
 the apply on the series is a separate and thorny topic, basically `lib.maybe_convert_objects` needs to be modified to return `timedelta64[ns]` if its detected, so this a separate issue. you can leave the tests in (but you can't really compare them), it 'works', but is the wrong dtype. Make separate issue (and separate PR) if you are so inclined.
 I'll defer for now. This is good enough to be usable.

If the latest build passes (it should) this is ready.
 gr8.....go ahead an create the other issue though (FYI look at the test in `tests/test_frame.py/test_operators_timedelta64` where I have exactly this problem (even works because it a mixed array)
 you are going to have to do the skip if np < 1.7 for most/all of those tests. you can possibly check if you should raise in the actual ops....in theory it should work, but not 100 %...numpy 1.6.2 is a bear
 I pre-emptively used `_skip_if_numpy_unfriendly()` on all of them. Seems to me, if you're using timedeltas on np < 1.7, you're asking your trouble.
 @jreback Looks like some datetime tests were broken. Will return to this tomorrow.
 @danielballan I did a PR to your branch to fix the last bit of tests....
 _I put this on your PR, @jreback, but I'm duplicating it here:_

One error left:

```
ERROR: test_timedelta_ops_with_missing_values (pandas.tseries.tests.test_timedeltas.TestTimedeltas)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/dallan/pandas-danielballan/pandas/tseries/tests/test_timedeltas.py", line 247, in test_timedelta_ops_with_missing_values
    actual = s1 + timedelta_NaT
  File "/home/dallan/pandas-danielballan/pandas/core/ops.py", line 473, in wrapper
    time_converted = _TimeOp.maybe_convert_for_time_op(left, right, name)
  File "/home/dallan/pandas-danielballan/pandas/core/ops.py", line 440, in maybe_convert_for_time_op
    return cls(left, right, name)
  File "/home/dallan/pandas-danielballan/pandas/core/ops.py", line 258, in __init__
    rvalues = self._convert_to_array(right, name=name, other=lvalues)
  File "/home/dallan/pandas-danielballan/pandas/core/ops.py", line 375, in _convert_to_array
    " operation".format(pa.array(values).dtype))
TypeError: incompatible type [timedelta64[ns]] for a datetime/timedelta operation

----------------------------------------------------------------------
Ran 829 tests in 43.246s

FAILED (SKIP=16, errors=1)
```
 where is this failing? the current pr passes
 My local copy wasn't quite right. Fine now. Ready to merge.
 ^ This is just a rebase to flatten.
 +1 on nice hisrory
