Builtins framework set up.
Messy commit. Fixed lots of user input sanitization problems (esp. with
pipe), and created two builtins.
Basic job control set up.
Improved builtin lookup mechanism.
Fixed bugs due to previous change in the builtin-lookup process.
Created basic done-job notification after next command.
Fixed problem with done-job status display after next command's finish.
Handled Ctrl-C.
Fixed bug for `cd`ing to a non-existant dir.
Merge branch 'jobs'

Conflicts:
	psh.py
Wrote questions' answers.
Wrote questions' answer. Attempted to un-zombify children, but didn't
manage to get it to work.
Updated docstring.
Readline utilized!!
Killed producer when consumer is no longer listening.
Basic ^Z handling set up.
Added signal-catching demo file.
Created de-zombify demo file.
Un-did all the job-related code.
Created JobList class.
"Broken" build. Process state related foundations ready, but job list is
broken, due to method lookup and deepcopying etc.