Update foreman to work with command mapping

The current implementation does not honor command mappings. For example,
adding foreman to the list of `bundle_bins` used by the
capistrano-bundler gem should prefix the foreman execution with `bundle
exec`, but this does not happen in the current implementation.

This commit fixes the issue by passing in the command as an argument
array rather than a single string. This allows `SSHKit::Command.new` to
properly identify `foreman` as the command and apply mappings
accordingly.
Use the logged in user as the default for foreman

In a vanilla installation of capistrano, no `:user` variable is ever set
which leaves `:foreman_user` undefined. This update defaults the
`:foreman_user` to the user used to login to the host, which should be a
more useful and generally accurate default.
Merge pull request #2 from devin-c/friendly_foreman

Update foreman to work with command mapping