Fix wrong assignation of invited_by field on  invitation is created.

Include:

- Django 1.7 now supports intermediate tables with more that one fk to same model. Because of it, this pr includes migration for change the old hack invited_by_id field to trully reference managed fk invited_by field.
- Moves the validation to serializer removing the adhoc create method on api resource.
- Fixes the wrong assignation of invited_by field (previously ignored)
- Increased the size of email and username field.
- WARNING: unique=True constraint is added to email field. This can be problematic if on production we have users with duplicate emails.