Cleaner UI for login and signup pages

![Screenshot 2014-08-15 17.51.52](https://dev.gitlab.org/uploads/dzaporozhets/gitlabhq/fc8bf3c4a4/Screenshot_2014-08-15_17.51.52.png)
![Screenshot 2014-08-15 17.52.39](https://dev.gitlab.org/uploads/dzaporozhets/gitlabhq/72706f55b0/Screenshot_2014-08-15_17.52.39.png)
![Screenshot 2014-08-15 17.51.33](https://dev.gitlab.org/uploads/dzaporozhets/gitlabhq/c46f972944/Screenshot_2014-08-15_17.51.33.png)

See merge request !1022