Add sample Capistrano deployment files

Add the following sample Capistrano configuration files to ease
deployment to a VPS using Capistrano:

 * `config/deploy.rb.sample`
 * `config/thin.yml.sample`
 * `Capfile.sample`

The sample Capfile will additionally load any recipes provided by
Discourse plugins.

Signed-off-by: David Celis <me@davidcel.is>