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`.
@arschles @taylorleese How about we don't start catching other exceptions, but we have these two return the same error message?