Add support for additional information on HABTM relationships

[mb5]

Twistar currently lacks support on adding information on HABTM relationship assosiations.
Consider the following example: An Actor plays roles in Movies. Each Actor can appear in multiple Movies, but play different Roles on each Movie. The DB table to represent this HABTM relationship would look like this:
Table Actor(name)
|
Table plays_in(movie_id)(role)
|
Table Movie(name)

Such a mapping is currently not possible with Twistar because the Role field cannot be accessed on a HABTM relationship, but it should be.