ForeignKey versioning fixes:

* fields inheriting from ForeignKey were not working properly (in reapply() for ex)
* saving new instance of versioned model with not null ForeignKey was resulting in "DoesNotExist" (as was comparing to empty object)
* display_diff() was not working with ForeignKey (was trying to set field='1' or 'None')

added set_field_data to be reverse for get_field_data.
added tests for this.
Merged in fsw_/django-versioning (pull request #2)

ForeignKey versioning fixes
fixed minor bug in tests
bump version 0.7.3, thanks to @fsw_