Fixes #8136: Add plugin documentation and updated rake tasks.

>Bastion has been designed to supply a common set of testing and development tasks and configurations using Grunt. These Grunt based tasks have been wrapped in Rake to make the Rails developer life easier. To setup your development environment, from your plugin's checkout:

Bastion supplies common testing and development tasks using Grunt. These Grunt tasks have been wrapped in Rake to make them more familiar to a Rails developer. To setup your development environment, from your plugin's checkout: