scheduling api changes and validation

- included 'schedule' type policy with 'at' and 'cron' in 'args'
- modified jsonschema and validating 'at' and 'cron' using custom formats
- using croniter package for validing cron entry
- added related tests
removed extra line after merge conflict resolve
simpler code for format checking

as per dried's excellent suggesstion
removed g_ prefix
added descriptions
Merge pull request #222 from rackerlabs/scheduling-api

scheduling api changes and validation

* policy 'type' can be schedule
* policy 'args' will contain either 'cron' or 'at'