Allow scheduling posts in a queue using new_post and an iCal RRULE

Add a --schedule switch to the new_post command, to be able to
schedule a post, based on an iCal recurrence rule.  Nikola can also be
configured to schedule all posts by default according to a specified
recurrence rule.
DOCS: Add docs for the scheduling feature.
Merge pull request #602 from punchagan/schedule-new-posts

Allow scheduling posts in a queue using new_post and an iCal RRULE