Added about_refinements.rb

This file demonstrates that modules can "refine" specific classes, giving them certain functionality if the modules are mixed into them, but not other classes that aren't enclosed by the "refine" keyword. This feature fits well with about_modules.rb, so it might make sense to include it in that file rather than in a file of its own (again, wrapping it with a test to make sure the user is using Ruby 2.0 rather than 1.9.x).

I still need to add the appropriate error to line 41, or merely changed fido.runs_fast to fido.respond_to? :runs_fast. However, there are still errors in this file if it is run, or if the methods and classes are extracted and run individually. I'll recommit when I fix these.

Thanks,
Stefan