Alert users of unsaved page changes.

Fixes #233 

### On link click
---
![screen shot 2014-02-20 at 2 30 00 pm](https://f.cloud.github.com/assets/96776/2224418/a032eb0c-9a7e-11e3-9e92-c2c1eb9239a3.png)

### On page refresh
---
![screen shot 2014-02-20 at 2 29 46 pm](https://f.cloud.github.com/assets/96776/2224433/ce1a8e08-9a7e-11e3-88bb-5f50dcfb0a82.png)

### On page close
---
![screen shot 2014-02-20 at 2 31 03 pm](https://f.cloud.github.com/assets/96776/2224438/e3d848a2-9a7e-11e3-88cb-930ac84eb012.png)