Plugins: add small plugin architecture

Making it easy to integrate any external action or api calls
into our workflow.
Github: add a method to get all pulls

Cache the result in a Class var for faster access the second time
around.
Plugins: add some more hooks

So plugins can react to more situations. Also, fix one
feature -> hotfix var name.
github.rb: DRY up some code, we have a method now

Also, don't bother passing in the api instance, we have a method for
that too.
Merge plugin-architecture (#117) into master

Plugins: add small plugin architecture

Making it easy to integrate any external action or api calls
into our workflow.