[#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)
[#4] Fixup logic on get

 - when run from ipython (outside of the test suite), I was getting
   KeyError instead of LookupError because the first guard on
   `pyconfig.Config.get` was incorrect.  I don't know why it behaved
   differently in the two environments.
 - Update README.rst docs.
fix format usage for python 3
Merge pull request #5 from yarbelk/master

add `allow_default` - permits loud failing of non existent settings