Add a preview script

 * Add a Gemfile to get correct Ruby gems for previewing
   (with "bundle install")

 * New preview script and preview config file

 * Update README and .gitignore

This way we will not have to edit _config.yml for
local previewing, and we should get a preview that's
closer to what shows up on the real site.

Signed-off-by: Don Marti <dmarti@cloudius-systems.com>