Improve legibility of diff views

New lines were displaying as thin strips. Spaces between lines made making comparisons more difficult. The +/- signs were hard to read against grey text.

* Give each line in the diff a minimum height, making new lines display thicker and maintaining a vertical rhythm
* Remove space between lines, the content naturally provides this spacing
* Extend red/green highlight to edges of the view
* Use darker red and green colours for the +/- icons

## Before
![screen shot 2014-12-16 at 11 25 21](https://cloud.githubusercontent.com/assets/319055/5452750/9f10905c-8516-11e4-91b3-4ee9db199d55.png)

## After
![screen shot 2014-12-16 at 11 25 37](https://cloud.githubusercontent.com/assets/319055/5452753/a38cfb98-8516-11e4-890d-3a19cedeb800.png)