Tweak npm package settings

Some refinements based on the [npm package.json specs](https://www.npmjs.org/doc/files/package.json.html).

- Lowercased the “T” in thoughtbot for `author`
- Added `author` URL
- Made homepage our Bourbon marketing site, instead of GitHub page
- Made `repository` URL point to actual Git endpoint
- Added `sass` keyword

Tested via these instructions: http://podefr.tumblr.com/post/30488475488/locally-test-your-npm-modules-without-publishing-them