Add image_optim gem
Run all of app/assets through image_optim
Updates README and CONTRIBUTING ref image optimisation
Merge pull request #479 from alxndrsn/image_optim

Image optim
Make image_optim development group only

We don't need this Gem to compile assets, it's for convenience only.
Merge pull request #480 from alphagov/make-image-optim-development-group

Make image_optim development group only