src/EventEmitter.js: don't use `inherits` module
EventEmitter: remove event initialization/validation
src/EventEmitter.js: define extensions outside of Object.create

This allows SIP.Utils.augment and jasmine's spyOn to work correctly.
EventEmitter: remove checkListener method

The rest of SIP.js uses:

  emitter.listeners(eventName).length

instead of:

  emitter.checkListener(eventName)
EventEmitter: remove off method

The rest of SIP.js uses removeListener instead.