alias global and temporary listener methods to "subscribe" for consistancy

responding to issue #66

Please note that I'm not that experienced with the gem, so I added a this PR draft to discuss and complete the issue.
I also suspect that the testing I've done is not sufficient.
and I'm sure that the testing is not DRY... maybe moving the specs of all the 3 methods (`.subscribe`, `.with_listeners`, `.add_listener`) to a [shared examples](https://www.relishapp.com/rspec/rspec-core/docs/example-groups/shared-examples) be better -which I plan to do before merging-