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
Is there a reason why the error_serializer is set with a method, rather than accepted as a param in the API constructor (much like request_type or response_type)?  Is it because request/response_type represent classes, while this is overriding a default behavior with a method instead of a class?