Checkbox sizes reflect the priority

The *bigger* the task, the *bigger* its checkbox.

Idea taken from the template at [The Daily Muse](http://www.thedailymuse.com/career/a-better-to-do-list-the-1-3-5-rule/)

Also, I aligned the text to the bottom of the checkbox, as it looked better to me. But, I consider myself to be a very bad front-end designer, so please correct it if needed.

This is how it looks on my system: -
![screenshot from 2014-01-25 18 08 52](https://f.cloud.github.com/assets/2862493/2001630/b4d3172e-85bd-11e3-98ee-c6b307fa1fd1.png)