Add plugins architecture

[jondeandres]

The plugins architecture will allow us organize the support for the different frameworks and libraries in a smarter way.

The idea is having all the external projects plugins in lib/rollbar/plugins. The files there will be loaded if the environment satisfy the dependencies that every plugin will define.

One plugin is defined in this way:

Rollbar.plugins.define('Sidekiq') do
  dependency { defined?(Sidekiq) }

  executes do 
    # here comes all the loading stuff, inject modules, monkey patches, add middlewares etc...
  end
end

If the dependencies are not satisfied the plugin is not loaded. My intention is migrate all the support for all the libraries into different plugin files/directories and cleanup a little our gem structure.