Plugins page

Based on a discussion with Lee [here](https://groups.google.com/d/msg/capistrano/ZFWVxOzsOZI/jJ7KFvAYjWcJ).

This pull request adds a plugins page under the 'Framework Extensions' nav section. First 4 plugins are added to the page.

![screen shot 2014-04-18 at 12 04 52 am](https://cloud.githubusercontent.com/assets/1042682/2737503/56b30974-c67c-11e3-99a1-269a67c000fa.png)