Added location_type :uri

This allows berkshelf to download gzipped tarballs from arbitrary urls.

This is done, in part, to support Gitlab (for public repositories).

I'll do a second pull request later for location_type :gitlab, which will
require credentials, etc. to download the tarball.

See [berkshelf-api #84](https://github.com/berkshelf/berkshelf-api/issues/84) for info on Gitlab.