Improve class counts for 'rake stats'

I've noticed that 'rake stats' does not a very good job of counting classes that have been reopened. I tend to use namespacing and inner classes quite a lot when writing Ruby so I end up with files like...

# some_lib/widget/helpers.rb
module SomeLib
  class Widget
    module Helpers
    ...
    end
  end
end

# some_lib/widget/extra_cool_stuff.rb
module SomeLib
  class Widget
    class ExtraCoolStuff
    ...
    end
  end
end

CodeStatistics would currently count SomeLib and SomeLib::Widget twice. This patch tries to do a better job of counting classes by using indentation within a file as a guide to namespace nesting. It also counts the number of modules and prints this in the output.