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 @taylorleese I'd like to merge the two since they appear to do the same thing. @2rs2ts `@` me after you push the change and I'll merge