Replace Timecop dependency with a simple stub
Explicitly reset Delorean after each test
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)