Add heading level class

For easier styling :

![capture decran 2014-03-01 a 01 47 01](https://f.cloud.github.com/assets/143380/2300846/a8142bc0-a10d-11e3-8464-4b2b2147ced2.png)

I did not used shorter `.h1`, `.h2` classes as is seems there is built-in styles in the atom browser for theses classes and would note produce a desired effect I think (36px on h1) :

![capture decran 2014-03-01 a 01 52 00](https://f.cloud.github.com/assets/143380/2300853/05217db8-a10e-11e3-9f66-94010fd1667b.png)