Horizontal headline slider

This PR adds a horizontal slider that allows you to change the starting x-axis of the meme's text by moving the slider: 
![image](https://cloud.githubusercontent.com/assets/1697151/3476447/e33eed58-02ff-11e4-8f20-10b719f16abc.png)

I'm also considering incorporating [draggable](http://jqueryui.com/draggable/) tags to allow for two-dimensional customization, but that would require adding jquery-ui and this is a quicker and easier fix.