BUG: timezone offset double counted using date_range and DateTimeIndex.append (fixes #2906, #2938)
fixes #2906, #2938

Does your patch fix a similar bug for concat in 10.1?

In [17]: from pandas import *

In [18]: dates_a = [Timestamp('20130219 13:00', tz='US/Eastern'), Timestamp('20130220 13:00', tz='US/Eastern')]

In [19]: dates_b = [Timestamp('20130222 13:00', tz='US/Eastern'), Timestamp('20130223 13:00', tz='US/Eastern')]

In [20]: a = Series(range(2), dates_a)

In [21]: b = Series(range(2), dates_b)

In [22]: concat([a, b])
Out[22]: 
2013-02-19 18:00:00-05:00 0
2013-02-20 18:00:00-05:00 1
2013-02-22 18:00:00-05:00 0
2013-02-23 18:00:00-05:00 1

Note that the index of Out[22] is wrong (time shifted by 5 hours).
 probably not, this was specific to date range generation...let me take a look
 this ok to merge?
 Looks fine to me-- I haven't looked to verify correctness but no tests had to be changed.
 yeah, it should be good
