Added pytz.timezone support.

This is for issue [#154](https://github.com/crsmithdev/arrow/issues/154) in which arrow did not support pytz.timezone being used.

Let me know if anything else needs to be done so that this can be merged.

I added tests, but always like to verify, just in case:
```
>>> import pytz
>>> import arrow
>>> str(arrow.utcnow().to(pytz.timezone("America/Los_Angeles")).datetime)
'2014-08-17 21:41:00.154590-07:00'
>>> str(arrow.utcnow().to(pytz.timezone("America/Los_Angeles")))
'2014-08-17T21:41:10.141368-07:00'
>>> str(arrow.utcnow().to("America/Los_Angeles").datetime)
'2014-08-17 21:41:57.354034-07:00'
>>> str(arrow.utcnow().to("America/Los_Angeles"))
'2014-08-17T21:42:03.748428-07:00'
```