Change JSON time format to 014-11-27T14:23:28.044Z.

Fixes #136.

__Note__: For the backend this is backwards compatible as we read the
old format as well but for the JSON API this breaks the format.
Merge pull request #143 from mvitz/136-fix-json-datetime-format

Change JSON time format to 014-11-27T14:23:28.044Z.