Finally get all test envs working (I think)
Clean up doc slightly.
Shooting in the dark.
_invalid would not be False on Python3.x

Some_LMDB_Resource_That_Was_Deleted_Or_Closed did not implement
__bool__, which is the __nonzero__ equivalent for 3.x. This caused
various tests to break.
Treat the database name like a Filesystem path.

If the database name is Unicode, encode it using the filesystem encoding
(probably ASCII). LMDB defines no encodings for database names.
Fix py33_cffi test environment definition.
Define and use portable bytes() helpers.

This supports Python2.5..3.3.
cffi Cursor.delete() default was Unicode, breaking 3.x.
Chagne test for next() existence.