Add group option to allow non-user groups.
Ignore empty lines in the Procfile.
Don't fail when upstart binaries are missing.
Don't fail when upstart binaries are missing for destroy_all_jobs method.
Put setgid if block on a single line.
Update specs with group option.
Add spec for nil/string values for group.
Merge branches 'group-option', 'empty-lines' and 'run-without-upstart'