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 :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