This reverts commit 33284014.

Reason: this change doesn't belong to DocRails.

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]

This reverts commit 7f536f91.

Reason: If at all this is done, this should be done in rails master.

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