Serialization: Requests now serialize requests between DCR models and JSON (not only strings)
Fixing GET endpoint: GET requests now looks up the parameter Graph ID and fetches the latest stored state from blockchain
Fixing Update endpoint: Update requests now looks up the parameter Graph ID, fetches latest stored state, executes supplied activity in the current graph state, gets new graph state and stores this on blockchain
Engine
Relation's Source and Target has been changes from being Activity objects to strings containing activity titles
Fixed bug in Graph's ID being auto-generated at every de-/serialization.
This PR includes fixes to: API
Engine
Resolves #14