Make jsonp support in revocation endpoint optional.

Also include the error in jsonp callback.
Client side creation of OAuth2 revocation requests.

Prepare the full request with url, headers and body. Also
support creating JSONP callbacks (included in URL query)
and the standard request utilizing request body.

Set default token type hint to access token for now. Will
consider changing to refresh_token if this is the more
common operation.
Sketching out an improved OAuth 2 client API.

The new API will be more consistent than the current and include a few
more security checks. This should reduce the work necessary for people
building OAuth 2 client support into their favourite HTTP request
framework.

Currently undocumented, untested and subject to change in the near
future.