Bundler enabled Capistrano provider

Based on discussion in #62. 

Creates a new provider that installs gems for `deployment` and `heaven` groups before running `cap deploy`. The `default` Bundler group is ignored. This allows multiple Capistrano versions, including 3.x, and plugins installed via gems to be used in deployments.

I left this as a separate provider to avoid unintended side-effects. Detecting Bundler groups is a bit too much magic for my taste, but if that is deemed to be desirable, I'll merge the providers to one and add logic to switch between the modes by reading Gemfile.