Cep18 rm

This PR builds upon #641, and should only be pulled *after* #641. 

#641 implemented CEP18 but did not remove legacy code. This PR removes that legacy code, namely MarketModel, Communicator, and Message. The Transaction class has been re-purposed to encapsulate the recording of transactions.

Note, @rwcarlsen, that this will change the output DB schema. I'd also like opinions on consolidating the transactions/transacted resources tables. In the updated paradigm, a single transaction is associated with a single trade, so they could (should?) be combined.

Also, @scopatz / @rwcarlsen, please let me know if you'd prefer the two PRs to be consolidated.