Adds support for renovating git references in `package.json` files. Supports updating semver tags or sha1 references ("commit hashes").

Also includes support for updating git references from *private* GitHub repositories, if the following conditions are met:
- Renovate user/token is authorised to read from the source private repository
- Dependency is written like `git+https://github.com/owner/repo.git#ref` in `package.json`

Closes #415, Closes #817