Use query parameter to pass referer

The reasoning for this pull request is explained in #233. The Pull Request includes an upgrade from Play 2.1.0 to Play 2.1.1 as well.

NOTE

The change is not 100% backward compatible. The routes file of a project using SecureSocial that includes this patch, will have to change two lines:

OLD
```
GET     /login                      securesocial.controllers.LoginPage.login 
GET     /signup                     securesocial.controllers.Registration.startSignUp
```

NEW
```
GET     /login                      securesocial.controllers.LoginPage.login(referer:Option[String])
GET     /signup                     securesocial.controllers.Registration.startSignUp(referer:Option[String])
```

Please verify the java sample application. I adjusted the routes file, but was not able to test it. All the scala parts are well tested and will be further tested in our company. We have this change rolled out to our dev environment already because we needed a solution for the issue.