Replace NSBGOnly with LSBackgroundOnly

The former is deprecated in favor of the latter.
Set deployment target to 10.7

I can't run it on my Lion machine otherwise. No code changes required.
Add NSRequiredContext

This should relieve the need for the user to turn the service on in
System Preferences, although I had a problem with that, probably
because of a plasma conduit breach.
Restarting apps isn't required

Newly enabled services show up automatically when the services list is
refreshed.
Recommend pbs

Nowadays, the user can run pbs instead of a custom
NSUpdateDynamicServices-calling program to refresh the services list.