Add plugin button to jumbotron

Considering all the other main menu items have a button in the jumbotron (homepage), the plugins page felt a bit left out ;-)

- Add plugin button
- Fix consistency of other buttons based on bootstrap pattern for buttons
- GitHub button is still not completely consistent, but that would mean the hover colour would be indistinguishable from the normal colour which would be beside the point.

For future reference: [this webpage](http://twitterbootstrap3buttons.w3masters.nl/) is a great resource to generate button colour combinations which are in line with the Bootstrap pattern.

![screenshot](http://snag.gy/Mntj2.jpg)