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:
looks like a formatting problem
I am writing a workaround on my project, will share
```
org.json4s.package$MappingException: Invalid date format 2014-07-17T12:09:01.320+01:00
	at org.json4s.ext.DateParser$.parse(JodaTimeSerializers.scala:63)
	at org.json4s.ext.DateTimeSerializer$$anonfun$$init$$4$$anonfun$apply$7.applyOrElse(JodaTimeSerializers.scala:68)
	at org.json4s.ext.DateTimeSerializer$$anonfun$$init$$4$$anonfun$apply$7.applyOrElse(JodaTimeSerializers.scala:67)
	at scala.runtime.AbstractPartialFunction.apply(AbstractPartialFunction.scala:33)
	at org.json4s.CustomSerializer$$anonfun$deserialize$1.applyOrElse(Formats.scala:364)
	at org.json4s.CustomSerializer$$anonfun$deserialize$1.applyOrElse(Formats.scala:362)
	at scala.PartialFunction$OrElse.apply(PartialFunction.scala:162)
	at org.json4s.CustomSerializer$$anonfun$deserialize$1.applyOrElse(Formats.scala:362)
	at org.json4s.CustomSerializer$$anonfun$deserialize$1.applyOrElse(Formats.scala:362)
	at scala.PartialFunction$OrElse.apply(PartialFunction.scala:162)
	at org.json4s.ext.ClassSerializer$$anonfun$deserialize$1.applyOrElse(JodaTimeSerializers.scala:121)
	at org.json4s.ext.ClassSerializer$$anonfun$deserialize$1.applyOrElse(JodaTimeSerializers.scala:121)
	at scala.PartialFunction$OrElse.apply(PartialFunction.scala:162)
	at org.json4s.ext.ClassSerializer$$anonfun$deserialize$1.applyOrElse(JodaTimeSerializers.scala:121)
	at org.json4s.ext.ClassSerializer$$anonfun$deserialize$1.applyOrElse(JodaTimeSerializers.scala:121)
	at scala.PartialFunction$OrElse.apply(PartialFunction.scala:162)
	at org.json4s.ext.ClassSerializer$$anonfun$deserialize$1.applyOrElse(JodaTimeSerializers.scala:121)
	at org.json4s.ext.ClassSerializer$$anonfun$deserialize$1.applyOrElse(JodaTimeSerializers.scala:121)
	at scala.PartialFunction$OrElse.apply(PartialFunction.scala:162)
	at org.json4s.CustomSerializer$$anonfun$deserialize$1.applyOrElse(Formats.scala:362)
	at org.json4s.CustomSerializer$$anonfun$deserialize$1.applyOrElse(Formats.scala:362)
	at scala.PartialFunction$OrElse.apply(PartialFunction.scala:162)
	at scala.PartialFunction$class.applyOrElse(PartialFunction.scala:118)
	at scala.collection.AbstractMap.applyOrElse(Map.scala:58)
	at scala.PartialFunction$OrElse.apply(PartialFunction.scala:162)
	at org.json4s.Extraction$.org$json4s$Extraction$$customOrElse(Extraction.scala:499)
	at org.json4s.Extraction$ClassInstanceBuilder.result(Extraction.scala:488)
	at org.json4s.Extraction$.extract(Extraction.scala:332)
	at org.json4s.Extraction$.extract(Extraction.scala:42)
```
I don't know why tests did not catch it!