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)

These were the changes needed to build the new <http://client.webfinger.net> which is now using this library.