Epic pagerduty improvements.

* use moment instead of datejs for time manipulation (ie adding
  hours/minutes)
* for `pager me N`, try to use the person who's said it email, instead
  of their name to find a PagerDuty user
* add command to remember your PagerDuty email address
* use moment's .calendar() to display nicer time when coverage is
* `pager me` will also say what it thinks your PagerDuty email address
  is
Move process.env.HUBOT_PAGERDUTY_SCHEDULE_ID to a variable