passing options instead of cb to spawnCommand

I was puzzled by why 'cb' was being passed to the spawnCommand method. I assume this was a typo? With the proposed change, you can now pass in options to the spawned command in order to do things like modify the PATH variable for the spawn'd process, etc.
I modified the assertions and removed the above condition. Regarding the `sinon.stub()` method, if the additional refactoring is a requirement in order to merge this change, I should be able to find time later this week to do so. Please let me know. Thanks!