Handle JsonMappingExceptions (issue #21)

Addresses #21 by adding a case for [`JsonMappingException`](http://fasterxml.github.io/jackson-databind/javadoc/2.4/com/fasterxml/jackson/databind/JsonMappingException.html) to `HttpResourceActor.handleError`.
@2rs2ts Perhaps you should just combine this and the previous case and match on ```JsonProcessingException```. That would catch them both and they are both 400's anyways.