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:
Odd -- it's parsing a valid date, right? The Scala client has test coverage for this stuff...