Fixes #4 and fixes #5.

Add a specific test case for both of these things.

Enabling the C extension on Python 3 revealed more places where
readonly attributes can raise different exceptions so fix those tests.

Also add Python 3.6 to the supported list, and use zope.testrunner to
workaround the namespace package issue.

The code is now in a "compatible subset" which supports Python 2.6, 2.7,
and 3.2, including PyPy 1.8 (the version compatible with the 2.7 language
spec).

Drop doctests, whose examples are now part of the narrative docs.