Plugins exception

To reasonably desume the plugins from the backtrace it is necessary to determine the root of the gem of the plugin. For this reason this patch changes the convention that the require file should be under the `./lib` dir (e.g. `./lib/cocoapods-plugin.rb`). This is coherent with RubyGems conventions and I don't expect related issues thus.

/c @alloy 

Closes https://github.com/CocoaPods/CLAide/issues/4.
`@plugin_paths ||= begin`...