Graphite-beacon

- Support systemd
- Update CLI handler
- Add PagerDuty handler

- Add Slack (https://slack.com) handler
- Add `request_timeout` alerts' option
- Change history_size format: 144 -> 1day

- Python 2.6 support

- Add `smtp.graphite_url` option for set graphite_url in emails
- Add `send_initial` option for send initial values when graphite-beacon starts
- Update HTML email templates

- Change format of handlers options