Rewrite and addition of boolean and date 'publishable' columns.

I've just checked in a major re-write of the publishable gem.  It allows not only datetime columns for the publishable field, but also date and boolean.  It also allows some additional scope features.  See documentation for details.

The gem has also been generally updated with better documentation (in YARD documentation format), RSpec tests, Bundler for dependency handling, etc.

I wrote this a while ago, based on the original [publishable gem](https://github.com/linki/publishable) and thought it should be shared back to the original, which doesn't appear to have been updated in quite a while.