Remove RBZ, give a generator instead

Instead of having an RBZ file in the repo, we can just have a script that generates one on request. This means that the repo size will stop climbing everytime the RBZ is updated and, as this plugin is aimed at developers, it can be assumed they will be able to operate the script. 

The solution I've provided is made to work with Ruby 1.8.6 (Same version that comes with SketchUp) so that developers can leverage the Ruby installation they likely already have. 

Adds two dependencies: rubygems and rubyzip2 (officially deprecated, but only available support for 1.8.6)