UUID support

[fommil]

Please add something like the following to BasicFormats: UUIDs are so widely used that they must be on par with some of the other types in that file.

trait UuidMarshalling {
  implicit object UuidJsonFormat extends JsonFormat[UUID] {
    def write(x: UUID) = JsString(x toString ())
    def read(value: JsValue) = value match {
      case JsString(x) => UUID.fromString(x)
      case x => deserializationError("Expected UUID as JsString, but got " + x)
    }
  }
}

[fommil]

see #25

[natbusa]

Awesome, it worked perfectly. Thanks! ( I might make a second version for datetime conversion). This conversions are really all over the places, it makes sense to extend the json (un)marshalling with your own set of traits.