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 Yeah, I noticed that as well but it shows up correctly in the databind javadocs. Your call regarding the change. I just wanted to point it out.