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
While I like that you're providing the facility to not serialize anything in a response, the method seems a little indirect.  You'd have to read the documentation well to know that this behavior exists.

However I can't think of an alternative other than requiring the caller to explicitly set a "serializable" field.