GitHubButton
Added OAuthPopupOpener.
UI.access() instead of Session.access()
OAuthPopupOpener demo
Updated README.md
Merge branch 'master' into github

Conflicts:
	oauthpopup-demo/src/main/java/org/vaadin/addon/oauthpopup/demo/DemoUI.java
GitHubApi redirect_uri
Changed callback url + request handler behaviour.

Now, "/oauthpopupcallback/ID" is added at the end of callback path
and the callback handler only handles such requests.