implement `be_monitored_by` matcher
return false if stdout is nil
Add specs
clear empty line
Merge remote-tracking branch 'pr/187/head'

Conflicts:
	lib/serverspec/backend/exec.rb
	lib/serverspec/commands/base.rb
	spec/debian/commands_spec.rb
	spec/redhat/commands_spec.rb
	spec/support/shared_commands_examples.rb