foreign keys on models with multiple primary keys

[devkral]

Changes

• rework ForeignKeys to work with multiple primary keys. Untested yet.
• add field_name for getter/setters
• fix #101
• use the descriptor protocol for many2many
• remove defunct (wrong type) and unused many_to_many collection

Missing Tests

[devkral]

Except the tests (multiple primary keys) I finished the cleanup of foreign keys and fixed the issue 101 on the way.

Do we want to merge it already? And continue to integrate tests later (I have also to provide some docs for ExcludeField)

[devkral]

oops, there is another bug.

I will finish it first