Associations: Added basic acceptance spec

I had to add some weired autoloading magic for the required fixtures of
the spec. Without that we would end up with some nice circular
dependencies. As far as I can tell it at the moment this is the reason
it will work in a Rails project. But this **not** yet confirmed.
DocumentModelMapper: Remember models with a reference
DocumentModelMapper: Remember which models are referenced
Updated .gitignore
WIP: Very first try to realize associations
WIP: At least no stack level too deep \o/

Currently the objects are not equal because the associations hold
different query object :-(