Support running a script as an alert type

When an alert is triggered for a service, the script at SCRIPT_PATH is
run with information about the service (including passing and failing
checks), the host, and the scheme encoded in JSON format.
Add ids, fix argument names in script alert arguments

- Added service and check ids to the passed json.
- Fixed overall_status spelling
- Fixed overall_status value (was sending constant)
Added extra script argument, plus service and hackpad url
Merge branch 'master' into alert_by_script
Updated API for script fields, fixed migration for script extra argument.
Merge remote-tracking branch 'baxter/alert_by_script'