Support for scheduling & overides

Porting @jnewland's work:

Introduce a couple new pager commands:

**Display the schedule for the next 30 days:**

`/pager schedule`

**Display any overrides for the next 30 days:**

`/pager overrides`

**Schedule an override:**

`/pager override 2014-07-10T16:00:00-07:00 - 2014-07-11T16:00:00-07:00`

**Delete an existing override:**

`/pager override delete <id>`

**Schedule an override for someone else:**

`/pager override 2014-07-10T16:00:00-07:00 - 2014-07-11T16:00:00-07:00 <username>`

This is in production and working. I'm going to use it real quick to accomplish https://github.com/github/ops/issues/2680, then deploy master and wait for review before merging.
Yeah, I think we can just sort by start time and be good :+1: