Medium editor css support

**NOTICE**: this PR includes code from #30.

Changes list: 

* Support themes from [Medium editor](http://daviferreira.github.io/medium-editor/);
* Support font-awesome;
* Plus-icon like on medium.com, with circle border and transition;
* Added theme switcher to demo page like on medium editor [demo](http://daviferreira.github.io/medium-editor/).

Examples: 
![](http://cl.ly/image/3w1Q1e1a3h2S/Screen%20Shot%202014-04-03%20at%2017.41.32.png) 

![](http://cl.ly/image/2N3y0H3B302u/Screen%20Shot%202014-04-03%20at%2017.38.58.png) 

![](http://cl.ly/image/1J3x13340e1J/Screen%20Shot%202014-04-03%20at%2017.38.51.png)