Add support for Capistrano 3.0.

This is the initial cut for Capistrano 3.0 support. I followed many of
the recipes provided by default in the new Capistrano versions.
Update puma.rake to fix default settings.

Instead of loading similar to a normal rake task let's use what
Capistrano seems to be doing in the official bundler gems. Namely there
is a namespace with a load default task.
Merge pull request #2 from johnbellone/master

WIP: Add support for Capistrano 3.0.