Support for multiple cursors

Closes #26

It expands for each cursor and enables us to edit them simultaneously:
![gifrecord_2014-06-21_170600](https://cloud.githubusercontent.com/assets/5219415/3349171/2ceeec5e-f956-11e3-894c-2f31381ac16c.gif)

It only expands if all cursors match the same snippet:
![gifrecord_2014-06-21_170741](https://cloud.githubusercontent.com/assets/5219415/3349174/3e4da53a-f956-11e3-9ba5-3a6bcceec96a.gif)

And it works from Snippets Available:
![gifrecord_2014-06-21_170822](https://cloud.githubusercontent.com/assets/5219415/3349177/4ae1715a-f956-11e3-8141-7f46b1bace84.gif)

Greets! 
Since you're destroying these selections, don't you want to clear out the `@selections` array rather than leaving destroyed selection instances in it?