Add support for RFC6750 Bearer Tokens.

Bearer tokens as defined in RFC6570 work very similarly to the way tokens are defined in grails-spring-security-rest.  With this update, a configuration setting can be turned on which will make the filter look for authentication "tokens" to be transmitted in the methods defined by the specification.  The filter will expect to see the token in the Authorization header as "Bearer <token>", or in the "access_token" query string parameter.

Because of design considerations, the third method of transmitting bearer tokens via form request body has not been implemented.
Add correct token rendering and failure response when using bearer tokens.

According to the RFC7650 specification, when a protected resource does not have
a valid token (or a missing token), the server should respond with a
WWW-Authenticate header.

When issuing a token to a client (upon successful authentication), the token's
format should match that which is defined in RFC6750 as well.

This resolves the remaining issues mentioned in #73.
Update tests to support new token JSON output format.

When refactoring the DefaultRestAuthenticationTokenJsonRenderer, a change was
made to the order in which fields are populated, resulting in the token always
being last.
Merge pull request #73 from ajbrown/bearer-token-support

Add support for RFC6750 Bearer Tokens.