Add support for types by specifying a schema parameter

It would be really handy to have support for types in a special field.
Some people also suggested it at the conference DjangoVillage 2014 in Orvieto (Italy).

I started to work on this feature because I really need it for this project: https://github.com/ninuxorg/nodeshot

My final goal is to be able for users of a web app to specify additional fields for their data without having to change the database.

The short term goal is to have a different ModelField which implements this feature.

I will try to finish it before the end of June. As usual, feedback is welcome.