Add `ProcessSignal` for sending signals to a Sanford process

This adds logic for sending signals to a Sanford process. This
will be used by the CLI to stop, halt or restart a Sanford
process. It will try and find an existing PID file for the process
and send the signal to the PID stored in it. This replaces logic
that currently exists in the `Manager`.

@kellyredding - Ready for review.