Adds put/get terminology and returns uid for put.

Hi Fabian,

As I explained in this ticket (https://github.com/hellp/saferedisqueue/issues/5), I think put/get is best suited for a queue than push/pop.
I also return the uid when you put an element in the queue since it can be useful for a reference.

Let me know what you think and if you are interested to integrate these changes.

Michael
Good point. It would make it consistent with what `get`/`pop` returns.