Update edition comparison view

This improves the diff view and brings it into line with that of travel advice publisher - it now shows a more detailed diff view that is calculated on the fly and highlights individual word changes between lines:

Before
![before](https://cloud.githubusercontent.com/assets/3687/4374277/04461fc6-4333-11e4-90bd-81b128ecb131.png)

After
![after](https://cloud.githubusercontent.com/assets/3687/4374280/088cce72-4333-11e4-8aea-148181ed6f91.png)

Story: https://www.agileplannerapp.com/boards/173808/cards/5970

I think it's fine. The '+' and '-' are now using the same colour pallet as the diff lines themselves. I personally don't think it's worth adding another two colours for these.