Update readme.md

Fixed the broken logo image.
Add Placeholder support for firefox 19+
Add helper for prefixed backface-visibility property
Update linear-gradient mixin to spec
Extracted function to helper file. Make linear-gradient function work to spec. WIP
Bring radial-gradient function and mixin up to spec
Make background-shorthand mixin play well with the new gradient changes
Finished gradients updates to spec. Refactored helper functions to helpers directory
Fix bug in sass prefixer
Depreacted background-size mixin
Remove ms vendor prefix from keyframes mixin
Removed some prefixes and other cleanup
Merge branch 'pl-linear-gradient-spec'
Version bump to v3.1.2
Borked rubygem. Forced to bump.
Merge master into gh-pages