[model] Account: add mtime (modification time) field
[serializer] DateTime: change all date time to ISO8601 fmt
[serializer] Account: serialise (but not de-) mtime field
DateTimeConverter: set timezone to UTC for new DateTime
[serializer] Use UTC for serialising DateTime
[ctrl.Abstracts] Add Last-Modified to get() based on mtime
[model] +StateLogEntry. Add log entries set to Abstract
[test/assets] Add StateLogEntries to all abstracts
[serializer] new StateLogWrites for StateLogEntry serialisation
[service] Abstracts: Add stateLog entries on create & update
[service] Abstract: +listStates to retrieve set of all state logs
[ctrl+route] Abstracts +listState @ /api/abstracts/:id/stateLog
[test] AbstractService: Add simple test for listStates
Merge pull request #111 from gicmo/log

Abstract: Add log for state changes, and mtime field 

Travis says OK