Resample discards timezone information
It appears that resample is now dropping timezone information on the index. Is this expected?

```
a = pp.Series(index=pp.DatetimeIndex(['2013-01-01 06:00', '2013-01-01 07:00', '2013-01-02 06:00'],
                                     tz=pytz.timezone('America/Los_Angeles')),
              data=[1, 2, 3])
b = a.resample('D', how='max')
print repr(a.index.tz)
print repr(b.index.tz)
print a.index.tz == b.index.tz
```

produces the following in 0.16.1. Notably the timezones for the original series and the resampled series are the same.

```
<DstTzInfo 'America/Los_Angeles' LMT-1 day, 16:07:00 STD>
<DstTzInfo 'America/Los_Angeles' LMT-1 day, 16:07:00 STD>
True
```

However that's not the case for 0.18.1. Caveat is that we're using the new resample syntax: `b = a.resample('D').max()`

```
<DstTzInfo 'America/Los_Angeles' LMT-1 day, 16:07:00 STD>
<DstTzInfo 'America/Los_Angeles' PST-1 day, 16:00:00 STD>
False
```
