Python 2 and 3 compatibility

- Drop PyCURL, since it's not Py3k compatible.
- Drop bitly_http module, just inline urllib(2) code.
- Steal *_types lists from six.
- Update isinstance calls.
- Update excepts.
- Decode response body as utf-8.
- Drop simplejson import, just use json
- Should be compatible with >=2.6.