Webview does not dismiss until process complete

The webview now waits to dismiss after the oauth code is posted
and the user has been fetched. This way you don't have an akward
period between when the webview modally dismisses itself and the
login stream completes.
Merge remote-tracking branch 'instructure/CanvasKit/master'
Merge pull request #38 from jarsen/master

Make webview wait to dismiss