Add flash message for /login and /register pages

When user is already authenticated, I've added a redirection on the profile page with the following flash message:

![capture decran 2014-01-23 a 14 16 53](https://f.cloud.github.com/assets/103900/1984663/b4dd6078-8430-11e3-9670-473f1f02f153.png)