Allow Ruhoh to load plugins using bundler

Hi,
to allow ruhoh to load plugins from gems I:
- required bundler in the ruhoh executable (only if gemfile is present)
- created a Plugin module that each plugin can include to use the "initializer" helper
- changed ruhoh.rb to execute all registered plugins

Each plugin is executed passing the Ruhoh instance as context.
I also moved the code that was loading the local plugins files in a plugin (LocalPluginsPlugin), I thought it could be a perfect example.

I added rspec to do some unit tests.