Move to_CSV inside the Graph class
Add Factories for special FactOrigin and test to_CSV

* has comma, newline and double quote in the string
* verify that it is serialized correctly
Rename Dbd::Fact::Fact to Dbd::Fact::Base
Rename FactOrigin::FactOrigin to FactOrigin::Base
Rename Graph::Graph to Graph::Base
Refactor to Graph::Base#collections
Fact::Collection#oldest_time_stamp
Adding 2 FactOrigin::Collections to a graph works
Move the Graph::Base#collections test to ../graph/collection_spec.rb
Addiing fact_collection to Graph::Collection works
ArrayCollection#last
Graph::Collection#newest_time_stamp
Graph::Collection#oldest_time_stamp