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
Hm, if I wanted to override the serializable criteria (i.e. not rely on a missing "title" to determine whether or not the error is serializable) then this would stop me.

I could come up with a contrived example (e.g. requiring every error have a title during logging of exceptions) but honestly if you don't want something to be serialized then omitting a title isn't a big deal.