1. Uses isabs() for program check which was mentioned here as a desired fix and makes sense.
2. Inserts program directory into position 0 in path search for program, which is similar to how path search works in other programs when executing a command from a directory.
3. Now has two tests for having the program in a custom path, and for having the program in the program directory.

Incorporates feedback from original pull:
#508

Resolves:
#369

I would update the docs. "If it is relative, the supervisord’s environment $PATH will be searched for the executable." => "If it is relative, the program's directory and supervisord’s environment $PATH will be searched for the executable."