Edit population title

## Story
* https://www.pivotaltracker.com/story/show/75134126

## Notes
* Needs more thorax goodness, less DOM hacking

## Screenshot

![image](https://cloud.githubusercontent.com/assets/2136286/4207709/6dacf7f8-3859-11e4-8875-f57043767973.png)

This is confusing because we're essentially updating the measure model, giving it a name and index, and only incidentally saving it to the population via some backend manipulation.