PagerDuty plugin can now ack & resolve incidents

The PagerDuty plugin has been upgraded considerably.

- `pagerduty open` lists open incidents.
- `pagerduty incident #` gives incident details.
- The plugin now uses the leveldb brain to store mappings from slack
user names to PagerDuty ids.
- `pagerduty users` lists its current user mappings.
- If a user has a pagerduty id, they can ack & resolve incidents by ID
from Slack.

A read/write pagerduty API key is now required.