Add failing tests and then fix them. Use a common function to DRY in
all the modules.

Fixes #78

Also use zope.testrunner in tox (and travis) because it allows passing
arguments to select what tests to run, see slow tests, etc.

Simplify setup.py to always build the C modules and avoid polluting
wheel caches (the same as zope.security and other projects that have
been worked on lately).

Bump the minimum 'persistent' requirement to one that actually works
in PURE_PYTHON mode (plus fix wheel metadata; otherwise we need
environment markers).

Test PURE_PYTHON in environments on Travis. Do some minor tuning to
eliminate redundant test cases and speed up test time.