BUG: comparing TZ-aware timestamps to a scalar results in exception

This works as expected:

```
pd.DataFrame({'test': [pd.Timestamp('2012-01-01 13:00:00'),
                       pd.Timestamp('2012-01-01 13:00:00')]}) == -1
```

However, this raises an exception:

```
pd.DataFrame({'test': [pd.Timestamp('2012-01-01 13:00:00+00:00'),
                       pd.Timestamp('2012-01-01 13:00:00+00:00')]}) == -1
```