implement tarball & zipball downloads; fixes #62

Reference zipball from https://github.com/hulu/restfulgit/zipball/7da1a61e2f566cf3094c2fea4b18b111d2638a8f
Reference tarball from https://github.com/hulu/restfulgit/tarball/7da1a61e2f566cf3094c2fea4b18b111d2638a8f