Install Mocha, for mocking and stubbing.

Decided overriding class instantiation manually will likely yield to
unnecessary pain. ;) Let's install Mocha and keep things clean(er).
Revert "Allow class overrides in Spy::Stats#initialize ."

This reverts commit c1eb1b3c4d2134c58fc7d2be9cb69d61207f8565.
Fix Spy::Stats tests, switching to use Mocha stubs.
Merge branch 'mocha' into develop