Add commit compare functionality

Hi,
This PR adds a view to compare two different commits. It also adds before and after commit SHAs and the compare URL to the webhook payload (similar to github). Most of the needed work was already done, so I just had to use most of what was already there :)

The url is of the form `user/repo-name/compare/beforeSHA...afterSHA`

It generates a page like this:

![image](https://cloud.githubusercontent.com/assets/28634/4112341/3abb266a-3225-11e4-809f-2b7f3a1e4ada.png)

with the diff defailts below.

Also I had to edit the main gogs css on this one. I didn't find any less files corresponding to that file...

Let me know any feedback and I will incorporate!

Thanks!

\- Chris