Tweak bower.json settings

Some refinements based on the [bower.json specification](https://github.com/bower/bower.json-spec) and to make it more aligned with the npm [package.json](https://github.com/thoughtbot/bourbon/pull/483).

- Added `description`
- Added `license`
- Added `keywords`
- Added `authors`
- Added `repository `
If we're using the `git` type, then the `url` should be `git://github.com/thoughtbot/bourbon.git`