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:
The changes I need to make it work on my side are:
```scala
implicit val formats = new DefaultFormats {
        override def dateFormatter = {
          val formatter = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss'Z'")
          formatter.setTimeZone(TimeZone.getTimeZone("UTC"))
          formatter
        }

        override val dateFormat = new DateFormat {
          def parse(date: String) = Try(DateTime.parse(date).toDate).toOption
          def format(date: Date) = dateFormatter.format(date)
        }
      } ++ JodaTimeSerializers.all
```