Timestamp subtraction should work for differing timezones
#### Code Sample, a copy-pastable example if possible

```ipython
In [1]: import pandas as pd
In [2]: pd.Timestamp("2016-01-01", tz="Europe/Berlin") - pd.Timestamp("now", tz="UTC")
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-691a8df26ecd> in <module>()
----> 1 pd.Timestamp("2016-01-01", tz="Europe/Berlin") - pd.Timestamp("now", tz="UTC")

pandas\tslib.pyx in pandas.tslib._Timestamp.__sub__ (pandas\tslib.c:23697)()

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

```
#### Problem description

If *both* timestamps have a timezone specified, the result of this operation is perfectly well-defined. It's quite surprising that I have to riddle my code with `lhs.tz_convert("UTC") - rhs.tz_convert("UTC")` lines to get the difference of timestamps.

#### Expected Output

    Timedelta('-393 days +06:29:07.057926')

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 58 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.19.2
nose: 1.3.7
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.1
numpy: 1.11.2
scipy: None
statsmodels: None
xarray: None
IPython: 5.1.0
sphinx: 1.5.1
patsy: None
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: None
openpyxl: 2.4.0
xlrd: None
xlwt: None
xlsxwriter: 0.9.6
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8.1
boto: None
pandas_datareader: None
</details>
