Timestamp subtraction of NaT with timezones
In pandas 0.17.1, a TypeError is returned when trying to subtract a timezone-aware timestamp from a NaT timestamp:

``` python
In [1]: import pandas as pd

In [2]: pd.Timestamp(None, tz='utc') - pd.Timestamp('now', tz='utc')
Traceback (most recent call last):

  File "<ipython-input-2-5e0738cec5fa>", line 1, in <module>
    pd.Timestamp(None, tz='utc') - pd.Timestamp('now', tz='utc')

  File "pandas\tslib.pyx", line 1099, in pandas.tslib._NaT.__sub__ (pandas\tslib.c:21618)

  File "pandas\tslib.pyx", line 1026, in pandas.tslib._Timestamp.__sub__ (pandas\tslib.c:20036)

TypeError: Timestamp subtraction must have the same timezones or no timezones
```

Subtracting a timezone unaware timestamp from a NaT timestamp is no problem. Also subtracting a NaT from a timezone aware timestamp works:

``` python
In [3]: pd.Timestamp(None) - pd.Timestamp('now')
Out[3]: NaT

In [4]: pd.Timestamp('now', tz='utc') - pd.Timestamp(None, tz='utc')
Out[4]: NaT

In [5]: pd.Timestamp('now', tz='utc') - pd.Timestamp(None)
Out[5]: NaT
```
