BUG: date parsers use different tz offsets
xref #12060

@jreback - my thought was to just convert the `dateutil` timezone offset (`tzoffset`) to a pytz offset?

``` python
# dateutil
In [1]: pd.Timestamp('2013/01/01 00:00:00+09:00')
Out[1]: Timestamp('2013-01-01 00:00:00+0900', tz='tzoffset(None, 32400)')

# C parser
In [2]: pd.Timestamp('2013-01-01 00:00:00+09:00')
Out[2]: Timestamp('2013-01-01 00:00:00+0900', tz='pytz.FixedOffset(540)')

In [3]: pd.date_range('2013-01-01 00:00:00+09:00', '2014/01/01 00:00:00+09:00')

TypeError                                 Traceback (most recent call last)
<ipython-input-3-d34c322923bc> in <module>()
----> 1 pd.date_range('2013-01-01 00:00:00+09:00', '2014/01/01 00:00:00+09:00')

...

TypeError: Start and end cannot both be tz-aware with different timezones
```
