Use of ISO time for date_range
This works:

pd.Timestamp('2014-01-01 00:00 Z', tz='US/Eastern')
=> Timestamp('2013-12-31 19:00:00-0500', tz='US/Eastern')

and this works:

pd.date_range('2014-01-01 00:00', periods=1, tz='US/Eastern')
=> <class 'pandas.tseries.index.DatetimeIndex'>
[2014-01-01 00:00:00]
Length: 1, Freq: D, Timezone: US/Eastern

but this does not:

pd.date_range('2014-01-01 00:00Z', periods=1, tz='US/Eastern')
=> assertion error

Is there a reason that I would not be able to create a date range with start time in UTC but to be used for another timezone?
