Change subclass of JSONFormField to CharField

This eliminates the problem where Django's model TextField passes parameters to the __init__ of JSONFormField which it can't handle by using a CharField's __init__. This is untested beyond working on my install, I've added no tests.
Merge pull request #83 from danostrowski/master

Change subclass of JSONFormField to CharField
Got tests running in Django 1.7
Merge branch 'master' of github.com:bradjasper/django-jsonfield
Added fallback for simplejson
Bumped version
Fixed bug with tests not running in older django versions
Added postgresql note in readme
Merge remote-tracking branch 'upstream/master'