Fixed Need to make checks that the callback is a proper function and eventName is a proper value as well #2

Fixed Null/undefined for param values to 'on' and 'once' methods.
After some thinking - we need to throw an error here instead of a silent return. Could you use `throw message` with some message saying that it is Events.js and passed input parameters are incorrect. You might want to modify test after, there is a way to do mocha tests for errors. Rather than that, this pull request looks great !