safer test for currently running app

The previous version could fail if the specified bundle was not installed.
Here we tell "System Events" until determining it is safe to do otherwise.
add caveats for Little Snitch

note - this currently is the only app cask that does not have
a "link" or "install" stanza
Merge pull request #2026 from rolandwalker/safer_quit_uninst

safer test for currently running app
Merge pull request #2050 from rolandwalker/little_snitch_caveats

add caveats for Little Snitch
Fix Contributing layout issue
service support