Implement GitHub

Adds support for GitHub in the form `foo/bar` or `foo/bar#tag` via the
GitHub archives
Merge branch 'tarball.js' into implement/github
Tests for github

One is telephoning to github so we might want to mock
this one with nock or something like this in future
set rimraf to 2.1.4
Merge pull request #4 from ForbesLindesay/implement/github

Implement GitHub