Add Post state options - Fixes #5

From https://github.com/gcr/tumblesocks/issues/5 

* Add "ask" as an option for post state; if "ask" is chosen, creating a post asks user to choose one of the allowed post states: published, draft, private, queue, or schedule
* Add "schedule" as an option for post state; if "schedule" is chosen, :state is set to "queue" and user is queried for a publish time. I don't really have any clue how Tumblr interprets the publish time text - during testing, things like "tomorrow, 5pm" worked, but I'm pretty sure you could also specify a fully specified time. See [tumblr's docs](http://www.tumblr.com/docs/en/advanced_post_options#schedule) to figure out what Tumblr accepts for the "Publish On" field. Posts created with "schedule" state can be found in the queue.