BUG: tslib.tz_convert_single converts DST incorrectly
Apply `tz_convert_single` to the same time representation with different tz should be all `True`?

```
import pandas as pd
import numpy as np
import datetime
import pytz

for i in range(9):
    utc_dt = datetime.datetime(2014, 3, 9, i, 0, tzinfo=pytz.timezone('UTC'))
    tz_dt = datetime.datetime(2014, 3, 9, i, 0, tzinfo=pytz.timezone('US/Eastern'))

    utc_ts = pd.Timestamp(utc_dt)
    tz_ts = pd.Timestamp(tz_dt)

    print(utc_ts, tz_ts)

    print(utc_ts.value == pd.tslib.tz_convert_single(tz_ts.value, pytz.timezone('UTC'),
          pytz.timezone('US/Eastern')))

    print(tz_ts.value == pd.tslib.tz_convert_single(utc_ts.value, pytz.timezone('US/Eastern'),
          pytz.timezone('UTC')))
```
## Current Result

```
(Timestamp('2014-03-09 00:00:00+0000', tz='UTC'), Timestamp('2014-03-09 00:00:00-0500', tz='US/Eastern'))
True
True
(Timestamp('2014-03-09 01:00:00+0000', tz='UTC'), Timestamp('2014-03-09 01:00:00-0500', tz='US/Eastern'))
True
True
(Timestamp('2014-03-09 02:00:00+0000', tz='UTC'), Timestamp('2014-03-09 02:00:00-0500', tz='US/Eastern'))
True
True
(Timestamp('2014-03-09 03:00:00+0000', tz='UTC'), Timestamp('2014-03-09 03:00:00-0500', tz='US/Eastern'))
False
True
(Timestamp('2014-03-09 04:00:00+0000', tz='UTC'), Timestamp('2014-03-09 04:00:00-0500', tz='US/Eastern'))
False
True
(Timestamp('2014-03-09 05:00:00+0000', tz='UTC'), Timestamp('2014-03-09 05:00:00-0500', tz='US/Eastern'))
False
True
(Timestamp('2014-03-09 06:00:00+0000', tz='UTC'), Timestamp('2014-03-09 06:00:00-0500', tz='US/Eastern'))
False
True
(Timestamp('2014-03-09 07:00:00+0000', tz='UTC'), Timestamp('2014-03-09 07:00:00-0500', tz='US/Eastern'))
False
True
(Timestamp('2014-03-09 08:00:00+0000', tz='UTC'), Timestamp('2014-03-09 08:00:00-0500', tz='US/Eastern'))
False
False
```
