Add Rex::Proto::PJL and PoC modules
Address @wchen-r7's comments
Address @jlee-r7's comments
Add filesystem commands and new PoC modules

This commit also refactors some of the code.
Refactor away leftover and redundant get_rdymsg
Remove newline from end_job to conform to spec
Replace magic numbers with constants
Refactor commands for modularity
Move {begin,end}_job from libs to modules
Make the module output prettier