Feature - Prevent clobbered timer globals from interfering with runner

`Sinon.js` exposes a functionality that mocks global timing functions. This change will save a reference to the global timing functions to prevent subsequent clobbering from affecting `runner.js`

See also: https://github.com/visionmedia/mocha/blob/master/lib/runnable.js#L10-L18

@chapel
@wpreul newline added