More django-ish form management

*Still work in progress*

* More Django-like form management (using `fields`, `exclude` and `readonly_fields`)
* `fields` determines the order of fields in the form and can also include methods on the model admin or on the document/model (as long as the methods are included in `readonly_fields`, too)
* Code cleanup (removes a lot of garbage that wasn't even used)
* Unit tests for all 3 backends + TravisCI configuration