Fixes #2 (maybe).

Previously, the extra required the 'ZODB3' metapackage:  depending on the
version used, the metapackage could pull in stale versions of **this**
package and 'persistent'.

I don't think we intentionally checked for Python 4.

Broeken version could contain garbage characters.

- Denominate files to indicate ReST.

- Drop .bzrignore.

Drop no-longer-used 'include'.

Note that the Python version exists only for BBB:  no application code
should instantiate one directly.

We need to set up an interpreteer copy which has 'persistent' already
installed, to prevent the on-demand eggs for 'persistent' and
'zope.interface' from screwing up the rest of the buildout.

Avoid relying on overflows w.r.t. sys.maxint.

Suppress warning under 'tox -e docs'