Alter the invert function to accept numbers, for native CSS invert
Merge branch 'invert_with_number' of https://github.com/iamcarrico/libsass into iamcarrico-invert_with_number
Passes tests, so merging.
Merge branch 'iamcarrico-invert_with_number'
Merge branch 'master' of github.com:hcatlin/libsass