store and retrieve messages for display in app

We really enjoy the messages feature but it was a bit of a shame that we couldn't read them after the push notification was gone.

With this PR the server stores all incoming messages to that they can later be retrieved by the apps, there is an accompanying PR on that repo, this is what it gives us:

![img_1169](https://cloud.githubusercontent.com/assets/749864/3656082/7c3017b8-1182-11e4-984a-c98d463adba2.PNG)

There is an [accompanying PR on the app](https://github.com/panicinc/PunchClock/pull/15) as well.