extract visible query out of Post.all

- allows Post.all to now include "all" (future and past) posts
- adds a visible option to where ex: visible => true will only select
  past posts
- add Post.visible to query all past posts
added option to `allow_preview`

- defaults to false
- if set to true future posts can been seen in the browser
remove todo: add syntax highlighting (already done)
change title to be more descriptive on allow preview documentation
remove double spacing
Improve wording of "future-dated posts" documentation
Merge pull request #24 from jess/preview

Allow Previewing of future posts option

* allow-preview:
  Improve wording of "future-dated posts" documentation
  remove double spacing
  change title to be more descriptive on allow preview documentation
  remove todo: add syntax highlighting (already done)
  added option to `allow_preview`
  extract visible query out of Post.all