Ability to set custom labels on buttons

Adding ability to set custom labels/icons for buttons. 

New option:
* buttonLabels: `false` | `'fontawesome'` | `{'bold': '<b>b</b>', 'italic': '<i>i</i>', 'anchor': 'link'}`

Option takes as an argument a string type name of the style or an object with label declaration. I've added pre-set styles for fontawesome, later could be extended to other icon-fonts. 

Also inside:
* New example;
* Updated docs;

![FontAwesome](http://cl.ly/image/2c3S3a1z1O3b/Screen_Shot_2013-12-09_at_18.38.20-2.png)
![Custom](http://cl.ly/image/2s471A0r2C1c/Screen_Shot_2013-12-09_at_18.38.31-2.png)


In response to https://github.com/daviferreira/medium-editor/issues/25

Not sure whether I took the best approach. Feel free to review.