Add field to scheduled job form to specify first/next run

I would like to add a new field to CRM_Admin_Form_Job that would let the admin/end-user specify the first/next run of the scheduled job. With the additional run frequencies proposed in CRM-17669, this would give much improved flexibility on when jobs are run.

My proposed implementation...

• Add the field in CRM_Admin_Form_Job::buildQuickForm(). The label for the field would change based on whether it's a new or existing job.
• Add the field to CRM_Admin_Form_Job.tpl with description explaining it's use.
• In CRM_Admin_Form_Job::postProcess(), if the field has been provided, use CRM_Core_BAO_Setting::setItem() to save the value.
• In CRM_Core_ScheduledJob::needsRunning(), use CRM_Core_BAO_Setting::getItem to check if there is a first/next run scheduled for the job. If there is, determine if the job needs to run based on that (and remove setting if so), otherwise fall through to existing check.