change annotate of initDomTreeTime and domReadyTime

[fromIRIS]

• api.domReadyTime
  i do not know the function of the domReadyTime, you say it measure the Time spending constructing the DOM tree. but the timing.domComplete means the time of total resources downloaded contaning images. and the timing.domLoading means the start time of constructing the DOM tree.
  by reading http://calendar.perfplanet.com/2012/deciphering-the-critical-rendering-path/ ,the timing.domInteractive is the end time of constructing the DOM tree, including the synchronous javascript time. so i think timing.domInteractive - timing. responseEnd is more suitful as a function "Time spending constructing the DOM tree".
• api.interactingTime
  besides, i think we can add a attribute 'interactingTime', it uses to measure the time user can interacting with the page,
  eg
  
  two numbers are almost same!