new approach for extending resque web with a plugin like system

So I took a new approach to trying to support plugins that seems like it might work nicely. 

Plugin authors can define a plugin under the `ResqueWeb::Plugins` namespace and then define an engine under that plugin. They have to define two class methods, `self.path_name` and `self.tabs`. The path_name is used to mount the engine inside ResqueWeb, and tabs is used to add tabs to the top.

Definitely needs some polishing up but seems like it might be the way to go. Curious if anyone has any criticism to offer. :)