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`.
@taylorleese That's fine as long as we don't mind that it technically includes [`JsonGenerationException`](http://fasterxml.github.io/jackson-core/javadoc/2.4/com/fasterxml/jackson/core/JsonGenerationException.html). Interestingly `JsonMappingException` is in the `databind` package (instead of `core`) so it doesn't show up as a direct known subclass in the javadoc.