Fix login pages

The login page looks fine in Firefox 26, but is not rendering properly in Chrome and Safari.  I added in some styling so that it is rendering across browsers properly.

https://rubygems.org/sign_in
https://rubygems.org/sign_up

![0k](https://f.cloud.github.com/assets/138048/1991898/b9bf783c-84a9-11e3-89bf-f09bc5e270d4.png)
![0j](https://f.cloud.github.com/assets/138048/1991899/b9bff5dc-84a9-11e3-8ab4-e894f7765c17.png)