Wrap MockFirebase in a UMD module definition and remove requireLib

Currently MockFirebase doesn't play nicely with Browserify because of its bespoke handling of ```require``` with ```requireLib```. This PR wraps MockFirebase in a UMD definition to resolve the problem.

All tests pass.
Hey @goldibex, thanks for this great contribution! I don't really understand how this line or the noConflict just below can work as-is (they refer to an empty exports object now). Thoughts here?