TIMESTAMP columns should include timezone in datetime dtype

[tswast]

I would expect the dtype for a TIMESTAMP column to be time zone aware.

In [1]: import pandas_gbq

In [2]: df = pandas_gbq.read_gbq('SELECT TIMESTAMP("2004-09-15 05:00:00") AS valid_timestamp')

In [3]: df.dtypes
Out[3]:
valid_timestamp    datetime64[ns]
dtype: object

Note: I believe the behavior used to be correct (include the timezone). NumPy 1.11 stopped defaulting to UTC and instead uses a naive datetime.

Related: see #69 for support for DATETIME columns (which do expect a naive datetime).

[tswast]

Fixed by #269