BUG: Arithmetic, timezone and offsets operations affecting to NaT
NaT affected by some datetime related ops unexpectedly.
# Arithmetic

Applying arithmetic ops to `NaT` is not handled properly. Based on numpy results, I understand that results should be all `NaT` as long as valid data is passed.

```
# current results
>>> pd.NaT + pd.offsets.Hour(1)
2262-04-11 01:12:43.145224192
>>> pd.NaT - pd.offsets.Hour(1))
OverflowError: Python int too large to convert to C long
>>> pd.NaT - pd.Timestamp('2011-01-01')
-734779 days, 0:00:00

# numpy
>>> np.datetime64('nat') + np.timedelta64(1, 'h')
NaT
>>> np.datetime64('nat') - np.timedelta64(1, 'h')
NaT
>>> np.datetime64('nat') - np.datetime64('2011-01-01')
NaT
```
# Timezone

Closes #5546. 

```
# current results
>>> idx = pd.DatetimeIndex(['2011-01-01 00:00', '2011-01-02 00:00', pd.NaT])
>>> idx.tz_localize('Asia/Tokyo')
<class 'pandas.tseries.index.DatetimeIndex'>
[2011-01-01 00:00:00+09:00, ..., 2262-04-10 00:12:43.145224192+09:00]
Length: 3, Freq: None, Timezone: Asia/Tokyo

>>> idx = pd.DatetimeIndex(['2011-01-01 00:00', '2011-01-02 00:00', pd.NaT], tz='US/Eastern')
>>> idx.tz_convert('Asia/Tokyo')
<class 'pandas.tseries.index.DatetimeIndex'>
[2011-01-01 14:00:00+09:00, ..., 2262-04-11 14:12:43.145224192+09:00]
```

_Note_ I fixed `DatetimeIndex`, and I leave `NatType` still doesn't have `tz_localize` and `tz_convert` methods `Timestamp` has. Is it should be added?
# Offsets

These have `apply` method which accepts `Timestamp`, but it cannot handle `Nat`.

```
# current result
>>> pd.offsets.Hour(1).apply(pd.NaT)
2262-04-11 01:12:43.145224192
```

Followings are results after the fix.

```
>>> pd.NaT + pd.offsets.Hour(1)
NaT
>>> pd.NaT - pd.offsets.Hour(1)
NaT
>>> pd.NaT - pd.Timestamp('2011-01-01')
NaT

>>> idx = pd.DatetimeIndex(['2011-01-01 00:00', '2011-01-02 00:00', pd.NaT])
>>> idx.tz_localize('Asia/Tokyo')
<class 'pandas.tseries.index.DatetimeIndex'>
[2011-01-01 00:00:00+09:00, ..., NaT]
Length: 3, Freq: None, Timezone: Asia/Tokyo

>>> idx = pd.DatetimeIndex(['2011-01-01 00:00', '2011-01-02 00:00', pd.NaT], tz='US/Eastern')
>>> idx.tz_convert('Asia/Tokyo')
<class 'pandas.tseries.index.DatetimeIndex'>
[2011-01-01 14:00:00+09:00, ..., NaT]
Length: 3, Freq: None, Timezone: Asia/Tokyo

>>> pd.offsets.Hour(1).apply(pd.NaT)
NaT
```
 pls rebase

needs a perf check - run a vbench and post/investigate >1.20 ratio
 Followings are vbench results. There seems to be no test constantly get worse, but is there anything should be checked?

First time

```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
....
frame_get_numeric_data                       |   0.2447 |   0.2143 |   1.1416 |
frame_fancy_lookup_all                       |  37.5026 |  32.1034 |   1.1682 |
timestamp_ops_diff2                          |  64.3330 |  54.9770 |   1.1702 |
timeseries_large_lookup_value                |   0.1390 |   0.0484 |   2.8719 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------

Ratio < 1.0 means the target commit is faster then the baseline.
Seed used: 1234

Target [9a66e5c] : BUG: Arithmetic, timezone and offsets operations affecting to NaT
Base   [c70b4ae] : BLD: change 2.7 build to use scipy 0.13.3
```

Second time:

```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
....
packers_write_pack                           |  12.9484 |  11.5690 |   1.1192 |
frame_dtypes                                 |   0.3934 |   0.3263 |   1.2056 |
frame_boolean_row_select                     |   0.5393 |   0.4443 |   1.2137 |
frame_to_string_floats                       |  82.4350 |  66.6873 |   1.2361 |
frame_assign_timeseries_index                |   2.0270 |   1.6250 |   1.2474 |
timedelta_convert_int                        |   0.4474 |   0.3517 |   1.2721 |
frame_get_numeric_data                       |   0.2870 |   0.2170 |   1.3227 |
```
 Got it. Modified and rebased.
 thanks
