BTree subclasses can define max_bucket_size or max_btree_size to
  control maximum sizes for bucket and tree nodes.
don't need to specify __slots__ (We're just continuing to not handle
slots and instance dicts correctly.)
Added documetation of node sizes and how to override them.
Python 3
Updated the Python implementation to match the C implementation.

Untested. I'll let travis take the first crack. Man, I need to
figure out tox.

Also, I'm not happy with the config variable names I originally
picked.  I'll fix those after the tests pass.
get rid of the "unreleased" version header.  It's best to decide the
version # when we're ready to release.
converted test MAX_SIZE
Merge remote-tracking branch 'origin/master' into node-sizes
don't look at max size unless we grew, since some tests don't set them
up.
try to make sure _Test classes have max sizes.
typo
sync split rule with C code.
renamed max_bucket_size and max_btree_size
fix typo
update from master
Merge pull request #8 from zopefoundation/node-sizes

Node sizes