Adding a script to gets visit stats from Google Analytics

Usage:

  hubot how many visits we had yesterday?
  hubot how many visits we had last year?

Response examples:
  Shell: Yesterday we had 121 sessions (59% new) and 274 pageviews. Average session duration: 00:02:35
  Shell: Last year we had 4138 sessions (53% new) and 9685 pageviews. Average session duration: 00:02:19