fixes for app engine, JRD marshalling, and draft-14

- add NewClient method which takes an http.Client used for outgoing
  connections.  This is needed for App Engine, since the standard
  http.DefaultClient is unavailable in that environment.
- specify JSON mapping for JRD type to get proper JSON marshalling
- accept new "application/jrd-json" mime type (relates to #1)
remove legacy fallback

Since that was the only thing using XRD, this removes the XRD package as
well.  Technically, it's still valid in the latest WebFinger spec to
request a non-standard content-type, but in practice no one does, so
there's little point in keeping support around.

This also makes the following changes to bring things in line with the
latest draft spec:
 - add support for application/jrd+json in addition to application/json
 - no longer require that the JRD Subject match the requested resource