Add `wait` example to docs.

[al6x]

Useful in specs, to wait for condition to became true,

it "job should be finished" ->
  startSomeAsyncJob()
  # Expecting job to be finished at some point in future.
  wait -> expect(isJobFinished()).to.eql true

  # It's also possible to implement more elegant version with promises, but it's 
  # incompatible with Node.js async convention.
  expect(isJobFinished()).to.eventually.eql true

# Wait until function called without exception.
waitTimeout  = 3000
waitInterval = 5
wait = (arg) ->
  if _(arg).isFunction()
    func = arg
    startTime = moment().valueOf()
    while true
      try
        return func()
      catch err
        currentTime = moment().valueOf()
        if (currentTime - startTime) < waitTimeout
          setTimeout sync.defer(), waitInterval
          sync.await()
        else throw err
  else
    time = arg
    setTimeout sync.defer(), time
    sync.await()