Replace Diffy with google diff match patch (dmp) js library

We are currently using diffy to display diffs, which is a wrapper around unix diff. There was feedback that editors would prefer not to see the duplicated lines, and see changes inline. This change achieves this as dmp allows you to tune the algorithm and the min match settings to display the differences inline.

Using the uncompressed version so that it doesn't break the asset pipeline.

![achieving strong and sustainable economic growth - policy - admin - gov uk 1](https://f.cloud.github.com/assets/215/1166844/404be16e-2083-11e3-918f-3991f262de47.png)