Fixed issues that were blocking support for Python 3 and Django 1.7.

utils.py
  - As Django as of 1.5+ supports Python 2.5 (which includes json) this should be removed and Django 1.7 does not have simplejson included.
  - force_unicode is only available to Python 2 environments. Includes force_text instead if Python 3 but uses an alias to prevent code branch logic where it's used.

widgets.py
  - urlpase is part of the urllib.parse library in Python 3.
  - Changed simplejson import to json
  - Alias basestring to point to str if Python 3. Doing this here prevents branching logic where it's used.
Merges pull request #22, adds some extra import checks to support Django 1.4