Support optional parameters for login

[tototoshi]

Currently, OAuth2Controller#login and OAuth2Controller#link doesn't support optional parameters.

play2-auth/social/src/main/scala/jp/t2v/lab/play2/auth/social/core/OAuth2Controller.scala

Lines 20 to 22 in 2e60a53

	// TODO scope is optional in some services
	// TODO some services have more optional parameter
	def login(scope: String) = AsyncStack(ExecutionContextKey -> OAuthExecutionContext) { implicit request =>

  // TODO scope is optional in some services
  // TODO some services have more optional parameter
  def login(scope: String) = AsyncStack(ExecutionContextKey -> OAuthExecutionContext) { implicit request =>

To solve this problem, I think some changes (including API changes) is required. My idea is:

• Remove the scope argument because scope is optional
• OAuth2Authenticator#getAuthorizationUrl should take request instead of scope

The task of login is only redirecting to authorization url, so it is enough only to change the API of OAuth2Authenticator#getAuthorizationUrl. is this correct?