Fix datetime string output to address firefox issue

This resolves issue #34

For some reason Tastypie (the REST API framework we are using) returns datetime like "2014-11-12 22:23:26". This makes some implementations of the Date() javascript function sad, e.g. Firefox which results in dates not displayed when fetched via the API. One example is event comments.

This CL makes the serialiser output the following format instead, which makes everyone happy:
2014-11-12T22:23:26+0000

(We are loosing some precision here, but that doesn't really matters because this is only used in some UI elements where second precision is just fine)

Note: The API code is due to refactoring and clean up. This will be done in a later CL (tracked in #9).
https://gist.github.com/berggren/3888c6ebb128f6adc63d

Reviewer: @joachimmetz

maybe use .endswith() instead.