Tweaks for compatibility with hood.ie

I've been working on adding BrowserID support to hood.ie using the browserid_couchdb plugin, and it required a couple of changes on the server side:

  * Hood.ie usernames are all prefixed with "user/", so the server needs to support a matching prefix
  * Hood.ie assigns special significance to the first item in "roles", so don't clobber it with the "browserid" role

(This is the first Erlang I've ever written, so I'm happy to take advice on how to do this better)