v0.15.0

• Added a 'first run' interface. If no users exist in the system at all,
  a12nserver will now drop you in a 'create admin user' interface, making the
  initial setup a LOT simpler.
• Updated the 'create oauth2 credentials' form to be more userfriendly.
• Fixed OAuth2 error responses.
• Added APIs for replacing the member list of a group (PUT).
• Added API for adding a member to a group (POST).
• Improved some of the installation instructions.
• Added a link from the users page back to the users list.