Avoid invoking a constructor

[rauno56]

At https://github.com/Rauno56/kue-chain/blob/master/index.js#L41 the parent job constructor is invoked to get the type of job to listen complete event on. It is bad in many sense.

Best solution which comes to mind is to create a factory function for constructing the different type of job prototypes and then save the type to somewhere on the object.