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 Disregard my last comment. What do you mean by "merge?" Like I said I'm not sure if we should catch other exceptions by matching on the superclass; particularly `JsonGenerationException` since that's more of an internal error, it seems:

>Exception type for exceptions during JSON writing, such as trying to output content in wrong context (non-matching end-array or end-object, for example).