[#4] Add loud fail for the get command(s)

 - Raise a key error if allowing default is expicitly prevented.  This
   should not break prior useages.
[#4] Add loud faile on Setting objects.

 - Settings now have the same interface as the basic config:
   allow_default=False raises an exception
 - Change the exception to more generic LookupException because it is
   not a KeyError, AttributeError or IndexError. (though attribute error
   isn't a subclass of LookupError, the other two are)