Call through callback when mocha emits end

This will fix #49 issue. Since mocha do not emits any events, if no tests was provided (see https://github.com/visionmedia/mocha/blob/master/lib/runner.js#L488) + callback is called in sync - we can't access to mocha runner from inside of the callback.
Merge pull request #56 from floatdrop/attach-on-end-event

Call through callback when mocha emits end