CSS clean up on style.css file

Hi,

I've made some clean up on this file :-)
My differents changes:

* Syntax homogenization (spaces, lowercase for hexa, ...)
* Code refactoring
* Simplification of some selectors
* Simplification of "0px" and hexa values
* More detailed font stack
* Add cursor pointer on ".bouton" elements

By the way, you should add your coding style guidelines (not only CSS) on the [CONTRIBUTING file](https://raw.github.com/inthepoche/poche/master/CONTRIBUTING.md).