Add Hyphens, Animation mixins to docs

* hyphens
* animation-delay
* animation-direction
* animation-duration
* animation-fill-mode
* animation-iteration-count
* animation-name
* animation-play-state
* animatino-timing-function
Add unpack function to docs
Strip-units function
Add em and rem functions
Remove dup unpack doc
Merge pull request #392 from thoughtbot/cr-new-doc-mixins

Add Hyphens, Animation mixins to docs