Use json4s-ext for date parsing

The client seems to have its own custom date parsing, at it appears a little funky, particularly on dates using [zulu time] (http://en.wikipedia.org/wiki/ISO_8601#UTC). This changes it to use the Json4s-ext library instead.

:pray: :mag:
Could it be related to this: https://github.com/json4s/json4s/issues/136