Split extension guide out of getting started guide.
Clarify instructions for mounting to non-root
Add spec tests for checking that the thumbnail strip option works
Add dragonfly image strip process for compatibility
Merge pull request #2560 from complistic-gaff/fix-image-split-option

Dragonfly::Register::NotFound (:strip not registered)
Merge branch 'master' of https://github.com/refinery/refinerycms