Add Reading Statistics to preview pane

Use [TextStatistics](https://github.com/cgiffard/TextStatistics.js) to provide the user with nice statistics that will help improve writing. It's possible to give more detail to these statistics by adding a way to see the ideal numbers.

![screenshot](https://www.dropbox.com/s/f11cphs8j4j62l9/Screenshot%202014-02-14%2017.41.04.png)