ENH/BUG: Offset.apply dont preserve time
Closes #7156.

@jreback Moved the description to API, and rebased.
 @jreback While trying to clean up the whatsnew page for 0.14.1, I wanted to rewrite the api change on the offsets a bit (as it is now not clear what did change), but I stumbled on this:

```
In [26]: d
Out[26]: Timestamp('2014-01-01 09:00:00')

In [27]: d + offsets.MonthEnd()
Out[27]: Timestamp('2014-01-31 09:00:00')

In [28]: d + offsets.MonthEnd(normalize=True)
Out[28]: Timestamp('2014-01-31 00:00:00')

In [29]: d + offsets.Day()
Out[29]: Timestamp('2014-01-02 09:00:00')

In [30]: d + offsets.Day(normalize=True)
Out[30]: Timestamp('2014-01-02 09:00:00')

In [31]: d + offsets.MonthOffset()
Out[31]: Timestamp('2014-01-02 09:00:00')
```

So for `MonthEnd` it seems to work as expected, but with `Day` `normalize` has no effect, and `MonthOffset` gives the same as `Day`. Are these bugs, or just due to my misunderstanding (I have to say that I almost never use offsets directly)
 cc @sinhrks ?
 Actually, looking at the dev docs now, and the example of `Day` is even in the docs (and so incorrect there): just above http://pandas-docs.github.io/pandas-docs-travis/timeseries.html#parametric-offsets

Correction: in the docs it is correct (with `apply` it seems to work correctly), but still the behaviour above seems strange.
 yes it looks like a bug with Day which is handled in Tick
 Yes, it is a bug. Currently `normalize` is effective only if addition or subtraction internally calls `apply`.  Fixed in #7697.
 And the `MonthOffset` issue?

And yet something else:

```
In [55]: offsets.YearOffset()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-55-b53e59b753d1> in <module>()
----> 1 offsets.YearOffset()

c:\users\vdbosscj\scipy\pandas-joris\pandas\tseries\offsets.pyc in __init__(self
, n, normalize, **kwds)
   1331
   1332     def __init__(self, n=1, normalize=False, **kwds):
-> 1333         self.month = kwds.get('month', self._default_month)
   1334
   1335         if self.month < 1 or self.month > 12:

AttributeError: 'YearOffset' object has no attribute '_default_month'
```
 can u separate out the fix for this into a new pr?
 @jreback Sure, will do shortly.

@jorisvandenbossche `YearOffset` looks an wrapper. It doesn't define effective `apply` and other methods. Maybe instanciation should be prohibited?
 @sinhrks In theory `YearOffset` should work like `MonthOffset`. open a new bug report for that pls.
 OK. Is it correct `DateOffset`, `MonthOffset` and `YearOffset` can be used in itselves (even though not included in `pd.offsets.__all__`). The result should be a timestamp increased one unit of the day, month and year?
 @sinhrks that's correct (I personally find that pretty confusing), as `ts + 1` does this.

so its a semi-private method (and one should really use `MonthEnd` et all)
 I created seperate issues for the YearOffset and MonthOffset issues I reported here. But do I understand correctly that they are not really 'meant to be used' like this?

As I expect `MonthOffset` to do the same as `DateOffset(months=1)` (and this _is_ something different as `MonthEnd`):

```
In [76]: d + offsets.DateOffset(months=1)
Out[76]: Timestamp('2014-02-01 09:00:00')

In [77]: d + offsets.MonthOffset()
Out[77]: Timestamp('2014-01-02 09:00:00')

In [78]: d + offsets.MonthEnd()
Out[78]: Timestamp('2014-01-31 00:00:00')
```
 @jorisvandenbossche its iffy. I don't see any explicty usage of them in docs/otherwise

I think the idea was that once could have it programatically select the class, e.g.

`DateOffset(months=2)` -> `MonthEnd(2)`

which does seem useful. Maybe need to figure out use/doc it (or clearly mark it as semi-private).
 I was just updating my comment above: `DateOffset(months=2)` is not the same as `MonthEnd(2)`! And I would expect that `MonthOffset` would be.
 sorry, I mean `Month(2)`, e.g. just advance me 2 months. 
 but `Month` does not exist? You only have `MonthBegin`, `MonthEnd`, `MonthOffset` in `pd.tseries.offsets`
 `Month` is de-facto `MonthOffset(months=n)`

The problem it is that 

`Timestamp('20130105') + pd.offsets.MonthOffset(months=1)` works! but `pd.offsets.MonthOffset(1)` is de-factor a 1-day increase as the default arg is `day`
 aha, so the `__init__` shouldbe overwritten in `MonthOffset`?
(and where is `Month`? I get `AttributeError: 'module' object has no attribute 'Month'`)
 I think the contructor is ok, it needs an apply (kind of like `MonthEnd` has , but w/o the month end stuff).

The question remains are `DateOffset/MonthOffset/YearOffset` public, or abc methods?
 I think this is related as well: https://github.com/pydata/pandas/issues/4804
 If we decide they are not public, they should get a `_` name. 
If it is public, I certainly think the constructor is not OK, the default arg should not be `day` for `MonthOffset`.

For the rest I can't really say if they should be public or not, I find all this offset stuff quite confusing, and the docs are rather scarce.
 ok, you have highlited the issues, I don't think its hard to 'fix' them to be correct. Let's do that, then maybe can expand the docs a bit / improve doc-strings.
