Tidy up diff between publication editions

* Reduce red and green contrast against black text, it was very hard to read. Use lighter colours.
* Put + and - in a column down the left side
* Don’t put diff in a well, use an empty bootstrap panel with a modified border

Before:
![screen shot 2014-07-04 at 18 35 54](https://cloud.githubusercontent.com/assets/319055/3484189/fbbe367a-03a1-11e4-92b0-bc43e836cf1b.png)

After:
![screen shot 2014-07-04 at 18 38 38](https://cloud.githubusercontent.com/assets/319055/3484193/11f69f5e-03a2-11e4-97e3-37048beee64f.png)