Add support for implicit OAuth 2.0 signin

[harrisj]

@vzvenyach has requested script access for logging into MyUSA so he can pull data from an authenticated backend. This isn't really feasible to do for the standard Authorization Code Grant Flow, since that requires a browser and an application running on a web server. But there is an alternate Implicit flow that would enable him to log in via OAuth. Here is an example of that code in the OAuth2 gem

auth_url = client.implicit.authorize_url(:redirect_uri => 'http://localhost:8080/oauth/callback')
# get the token params in the callback and
token = OAuth2::AccessToken.from_kvform(client, query_string)

It sends a URL, but the access token is included in the auth_url response and there is no need to visit the URL to get the access token. Instead, it can be pulled directly from the auth_url query string.

This will require only a slight tweak to doorkeeper.rb to enable. Are there any problems if we make this change at this point. There are less elegant alternatives should it be necessary, but I think this change is easy enough if it's okay.