Added first implementation of an Identity Map
Integrated the Identity Map into the loading process
Added a Session middleware to reset the IdentityMap
Added another acceptance test for Identity Map
Refactored the Identity Map implementation
Reek: Enabled :clock12::gun: for DocumentModelMapper#document_to_model
Added some documentation to the IdentityMap
YARD: Enabled markdown for YARD
Rubocop: Exclude the spec/setup (again)
Merge pull request #38 from triAGENS/introduce-identity-map

Implement an Identity Map