Cleanup changelog and add new tab.

:phone::feet:

Follow on from https://github.com/zendesk/samson/pull/188

This is still a WIP (I'll try add some more polish) but feedback is welcome. I'll check if other formats would be useful (e.g. Confluence markdown) tomorrow. I'm open to suggestions on how to add that. 

BTW, I had to lookup the syntax for an `img` tag I'm that rusty at front-end work :smile: 

* Cleanup changelog page
* Add new `Changelog` tab

![screenshot_10_11_2014_22_11](https://cloud.githubusercontent.com/assets/527473/4984729/926229f8-6926-11e4-9808-f2db15cff214.png)

Click that you get this: 

![screenshot_10_11_2014_22_07](https://cloud.githubusercontent.com/assets/527473/4984673/0f7e8bc6-6926-11e4-9b60-4464836deb8a.png)

/cc @steved555, @dasch, @jwswj, @halcyonCorsair, @princemaple, @bolddane, @pswadi-zendesk

### Risks
 - None
http://caniuse.com/#search=border-radius :)
Safe to leave these out.

FYI, if you want to do vendor prefix, you have to put them above the standard one.