BUG: concat of objects with the same timezone get reset to UTC
http://stackoverflow.com/questions/24830952/changing-timezone-on-pandas-datetimeindex-when-concatenating-dataframes-in-pytho

```
ix1 = pd.DatetimeIndex(start=pd.Timestamp('20140715', tz='EST5EDT'), end=pd.Timestamp('20140717', tz='EST5EDT'), freq='D', tz='EST5EDT')
ix2 = pd.DatetimeIndex([pd.Timestamp('2014-07-11 00:00:00', tz='EST5EDT'), pd.Timestamp('2014-07-21 00:00:00', tz='EST5EDT')])
df1 = pd.DataFrame(0, index=ix1, columns=['A', 'B'])
df2 = pd.DataFrame(0, index=ix2, columns=['A', 'B'])
```

I think the because the 2nd index is only length 2 its freq is not inferred causing a reset to UTC.

```
In [46]: concat([df1,df2])
Out[46]: 
                           A  B
2014-07-15 04:00:00+00:00  0  0
2014-07-16 04:00:00+00:00  0  0
2014-07-17 04:00:00+00:00  0  0
2014-07-11 04:00:00+00:00  0  0
2014-07-21 04:00:00+00:00  0  0

In [47]: concat([df1,df2]).index
Out[47]: 
<class 'pandas.tseries.index.DatetimeIndex'>
[2014-07-15 04:00:00+00:00, ..., 2014-07-21 04:00:00+00:00]
Length: 5, Freq: None, Timezone: UTC
```
