Notifications via PagerDuty and Mailgun

- First stab at issue #3
- Add support for notifications via mailgun api
- Add support for notifications via Pagerduty incident api. Support's
  triggering and resolving events. Also allows grouping by host or
  host:check_name for incident key.
Add email in addition to PagerDuty and Mailgun.

- Add's email notification via smtplib
- fixes #3