add support for posting 'when'

allow for back-filling of events during posting by manually setting 'when'.

e.g. curl -X POST http://localhost:8000/events/ -d '{"what": "deploy that thing", "tags": "deploy", "when": "1411142400.0"}'

p.s. apologies for the bad style, I wasn't sure how best to fit that there matching the existing style.