Rename gradient functions to allow writing valid CSS

Currently the gradient functions override the spec CSS gradient functions and prevent a user from writing valid CSS. This renames the `linear-gradient` and `radial-gradient` functions to `generate-linear` and `generate-radial`.

I split the PR into two commits — I'm assuming there's a build step to create dist/ but I didn't find any documentation, so edited manually.  

Bourbon.io docs will have to be updated too :smile:.

Fixes https://github.com/thoughtbot/bourbon/issues/177.