overlay.py: Adds self.ovl_type

To allow a stub function to tell the user the current overlay type
of the unsupported overlay, this variable will be passed in along
with the parent variable.
Adds stub module

In the event that a user doesn't install layman with a specific
overlay type, the StubOverlay class will be used.
api.py: Makes supported_types() check for modules

Prior to executing require_supported() and notifying a user that the
overlay type isn't supported due to not finding the proper command
bin, a check has been made to see if the overlay module has been
brought in by the user. This will prevent unnecessary complaining from
layman.