remove default realm so we can use empty realms

right now there is no way in the UI to support an empty realm because the uri is used to generate a default realm.  no where in the oauth specifications does it say a realm should default to any specific value and it does explicitly say realms are optional.

http://tools.ietf.org/html/rfc2617#section-1.2
http://tools.ietf.org/html/rfc5849#section-3.5.1