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:
On another code base, this line seems to broke extraction. Code compiles but execution hangs:
scala
```
val lastModified = (json \ "response" \ "content" \ "fields" \ "lastModified").extract[DateTime]
```

any idea?