Django 1.7 Compatibility

- Tries to use standard library json instead of Django's bundled simplejson (which has been removed in 1.7).
- Uses HttpResponse's content_type argument instead of mimetype (removed in 1.7).
- Adds deconstruct() methods to this library's custom field classes; this is needed for Django 1.7 migrations, as both of these field classes are not properly handled by their superclass's deconstruct() implementation.
- Refactors views.py for less duplication
- Style (PEP8) fixes