issue with changing frequencies when timezones are involved
the following code fails:

```
dr = date_range('2011-12-01','2012-07-20',freq = 'D', tz = 'US/Eastern')
dr.asfreq('T')
```

it appears that the asfreq code checks whether the timezone of the start and end date are the same. In this particular example, the daylight savings offsets are different at the beginning and end so the test for equality fails. Is this intended behaviour or a bug?
