Add process tagging for scheduler process

On a server hosting multiple apps and multiple instances of
resque-scheduler, it can be difficult to tell which scheduler
goes with which app. This change adds an APP_NAME environment
variable that, if set, appends the app name to the procline.
allow customization of lock key via env var
Merge branch 'custom-lock-key'
Merge remote-tracking branch 'nmeans/process-labeling'