Support multiple schedules

Removes the use of the `HUBOT_PAGERDUTY_SCHEDULE_ID` environment variable and adds the following functionality:

* List all schedules with `hubot pager schedules`
* Respond with all schedules when asked `hubot who's on call`
* List the upcoming entries on a named schedule with `hubot pager schedule <schedule>`
* Perform overrides on individual schedules with `hubot pager me buddy 60` or `hubot pager override buddy $TIME $LATER_TIME`

The commonly used deprecated versions of these commands (`hubot pager me 60`) have usage messages pointing folks to the new version to ease the transition. Since this is a breaking change, we should call this 2.0.0 I guess.

Most commands expect schedule names to not contain spaces. I think that's a fine tradeoff.

/cc @technicalpickles 
Would it be worth documenting this in the README? Like, a hacking section?