Add functionality to save transaction response to db.

Update transaction record to contain response.
Update transaction status from "Processing" to "Completed".
Message store

Clean up response store code.
Add test assertions to response store test.
Refactor response test.
Merge branch 'messagestore' of https://github.com/jembi/openhim-core-js into messagestore

Conflicts:
	src/messageStore.coffee
	test/messageStoreTest.coffee