TimezoneType should be Unicode, not CHAR(50)

I'm getting this error when loading time zones:
```python
UnknownTimeZoneError: 'America/New_York                                  '

len('America/New_York                                  ')
Out[6]: 50
```

When process_result_value is called [here](https://github.com/kvesteri/sqlalchemy-utils/blob/122f6123260a601c2001f5878bb72bcf1c2691cb/sqlalchemy_utils/types/timezone.py#L85), pytz throws an exception, as the trailing spaces throw it off.

Looks like the culprit is that the column type is a `CHAR(50)` (which forces a 50 char column) while it really should be simply `Unicode`.

This pull request fixes the issue for me.