profiles can link to gittip.  api changes for gittip integration.

Users can add a gittip username to their profile, which will cause
their profile page and the gem pages for any gems they own to link
to relevant pages on gittip.

As well, some API changes were needed so gittip could integrate
effectively.  These include:
  1) gems/:gem_name/owners.:format now includes the gittip_username
     when the relevant profile has opted in.
  2) profiles/:username.:format now exists, and returns profile info.