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
DocumentModelMapper: First outline to realize associations
DocumentModelMapper: At least no stack level too deep \o/

Currently the objects are not equal because the associations hold
different query object :-(
Query: Implemented #== method
DocumentModelMapperSpec: Added FakeIdentityMap to constructor
CollectionSpec: Added spec for creation of associations
DocumentModelMapper: Added specs for #model_to_document
Extracted the proxy classes in dedicated files
DocumentModelMapper: Added specs for using the proxies
Proxies: Using the collection_for method inside the proxies
Clean up code for metrics
Guacamole::Proxy: Added some basic documentation