New: toolbox/proc; Updated README

toolbox.proc now incorporates the methods isRunning(process) and
executeOnce(process) used to be included in <hooks>.
Trying to separate any logic that is not internal to qtile and
bundle it in toolbox for recycling and maintainance purposes.

<README> updated to reflect some changes.
<+TODO+>Include all commands in <README>
__accidental push__