Prettier alternative login buttons

I have made some small css changes that increase the width of the alternative login buttons to 100% (minus margins) and centres the button text.

before:
![before](https://f.cloud.github.com/assets/1328243/333553/f20dc6c8-9c59-11e2-9a89-6a2ce4d77f5e.png)

after:
![after](https://f.cloud.github.com/assets/1328243/333554/f69dd1ba-9c59-11e2-8ddb-faf6afef3fbd.png)

I have tested this only with Firefox 19.0.2 and Chrome 26.0 on Ubuntu 12.04, but it should work with all browser versions that aren't completely outdated.

Do I really have to sign the contributor agreement, before you pull these changes?