Add support to cron expressions.

[Horusiath]

List of various examples can be found here. This would probably need to have a more specialized version of Schedule message that accepts cron expressions instead of date/interval pairs.

Cron grammar:

Field name	Mandatory?	Allowed values	Allowed special characters
Seconds	No	0-59	* / , -
Minutes	Yes	0-59	* / , -
Hours	Yes	0-23	* / , -
Day of month	Yes	1-31	* / , - L W
Month	Yes	1-12 or JAN-DEC	* / , -
Day of week	Yes	0-6 or SUN-SAT	* / , - L #
Year	No	1970–2099	* / , -

API Proposal

var task = new Reminder.ScheduleCron(taskId, recipient.Path, "message", "0 */30 * ? * *")