Added support for queued posting

New rake tasks:
    - *new_qpost['Title']* - adds a new post to the tail of the queue
    - *new_qhpost['Title']* - adds a new post to the head of the queue
    - *publish_next* - publishes the next post from the queue by
      copying it to the _posts directory and renaming it with the
      correct date for today.
    - *new_post['Title',date] - adds a new post with the given date to
      the regular _posts folder.

New config options:
    - *queue_dir* - directory for the post queue (including source
      files)
    - *queue_file* - yaml file for the post queue

Added config option for using the `--no-future` option in Jekyll, so
future dated posts are not published by default (thanks to
https://github.com/jbrains/octopress/commit/4b9b499251ea4d613f7bd6f3b0500c6485c5ec21
for most of the code for that).