lib/on.js
function on (state, eventName, handler) {
  state.emitter.on(eventName, handler)

  return state.api
}
function on (state, eventName, handler) {
  state.emitter.on(eventName, handler)

  return state.api
}
function on ( state , eventName , handler ) { state . emitter . on ( eventName , handler ) return state . api }
binds event to handler

@param  {String} eventName   push, pull, connect, disconnect
@param  {Function} handler   function bound to event
@return {Promise}
binds event to handler