Add diff view for changed titles and description texts - Ref #413

This will add a diff algorithm in PHP and use it to render differences in titles and description texts.

Example:

![debug-3874](https://cloud.githubusercontent.com/assets/33978/3897329/c9c6ffca-2260-11e4-983c-065a51929659.png)

This is similar to what GitHub does:

![debug-3875](https://cloud.githubusercontent.com/assets/33978/3897332/cd592140-2260-11e4-8957-6739b81be5e1.png)