Compressed all images from the guide through ImageOptim which saves sometimes up to 74.6% in size [ci skip]

Revert "Compressed all images from the guide through ImageOptim which saves sometimes up to 74.6% in size [ci skip]"

Reason: this change can't really be done in docrails,
see http://weblog.rubyonrails.org/2012/3/7/what-is-docrails/

This reverts commit bc06e8d7.

Compressed all images from the guide through ImageOptim which saves sometimes up to 74.6% in size [ci skip]

If you're a guide writer and want to generate the guides, Run :

  ruby railties/guides/rails_guides.rb

And guides HTML will get generated inside railties/guides/output directory.