date_range with a given timezone fails if the start argument also has a timezone.
I am using pandas version 0.11.0.dev-eb3134f

This code will raise [this assert](https://github.com/pydata/pandas/blob/master/pandas/tseries/index.py#L303-304)

My debugging shows that the code is testing the equality of a timezone object (inferred_tz) to a string (tz).

date_range si supposed to receive a string so I think that it is a legit use of date_range.

``` python
from pandas import *
start = Timestamp('20130220 10:00', tz='US/Eastern')
dr = date_range(start, tz='US/Eastern')
```
