add guard-rubocop + default rubocop config
MASSIVE CLEANUP: fix ~ 700 rubocop offenses
Stub Kernel.system and Kernel.`

Use RSpec examples metadata feature to not stub these methods when the
:no_system_stub metadata is present.
Introduce Guard::Sheller to replace system/` calls.
complete Sheller + add stderr
stub all remaining system() calls with Sheller
add missing notifier stubs
Merge pull request #602 from guard/massive_cleanup

Massive cleanup