Slightly more elegant and robust buttons layout

Use centering and horizontal margins instead of assuming always two buttons and using "float" in addition to margin-right and left.

Before:

![before1](https://f.cloud.github.com/assets/326335/1405382/5ad0ec16-3d28-11e3-9eb1-fd1bcbef1e71.png)
![before2](https://f.cloud.github.com/assets/326335/1405383/5aeadf40-3d28-11e3-90d0-bdcde808b317.png)

After:

![after1](https://f.cloud.github.com/assets/326335/1405380/5a8dfa32-3d28-11e3-8e6c-6e9ed0def57d.png)
![after2](https://f.cloud.github.com/assets/326335/1405381/5aaf9a2a-3d28-11e3-9b33-00ce8a317145.png)