Capistrano 3 improvements

* Fix that a custom whenever command couldn't be defined unless you set `:whenever_command` to `:whenever` and then set `SSHKit.config.command_map[:whenever] ` to your custom command.
* Add `--roles` option that didn't make over from the Cap 2 port.
* A dash of housekeeping.

This would replace #417.
cc: @betesh, @phallstrom