Added Acceptance spec for graph bases relations
Added basic edge implementation
DocumentModelMapper: Added an Attribute class to represent mapped attributes
Updated Rspec to 3.1

This was done because I needed the `have_attributes` matcher
DocumentModelMapper: Extracted handling of references/embeds
Mapper: Add proxy for relations within a graph
Configuration: Added the graph to the config
Relation Proxy: Added actual implementation
DocumentMapper: Remove related attributes from document
WIP - Collection: Attempt to use transactions for Relations