add subscribeOnce alias in mediator
add same aliases for all backbone event subscription methods
add subscribeEventOnce to the EventBroker
remove arguments type checking
make mediator.subscribeOnce method readonly
add tests for mediator.subscribeOnce and eventBroker.subscribeEventOnce
Merge pull request #802 from YAndrew91/mediator-subscribe-once

Add mediator.subscribeOnce and mediator.on/once/off/trigger