Added gmock (Google Mock) package to complement gtest package.

I needed to use gmock and gtest together in a project, so I more or less copied the gtest package into a gmock package.

Another thing to do might be to rewrite the synergy package to use this gmock package, as it downloads gmock itself. But for now here's a standalone gmock package.
"Don’t include a period at the end. […] For brevity, don’t repeat the name of package — just describe what it does."
See http://nixos.org/nixpkgs/manual/