feat(HTTPError): Support custom media types for error responses

This patch adds support for serializing errors to other media types
besides the generic JSON and XML formats. It does this in two ways:

1. The default serializer will check for a '+xml' and '+json' suffix
   in the media type requested by the client, and if present, serialize
   to XML or JSON, respectively. This was thought to be better than
   simply returning an empty body in the response.

2. The default serializer can now be overriden with a custom
   serializer function, which affords complete control over the
   error serialization process.

Closes #265
Yeah, I think it may actually be better to have a more explicit "serializable" property. This patch was just building incrementally on the previous code (which used None to trigger whether or not to have a blank body), whereas it should have just gone all the way and cleaned up that cruft.