* Extended DB & DBEnv set_get_returns_none functionality to take a
"level" instead of a boolean flag. The boolean 0 and 1 values still
have the same effect. A value of 2 extends the "return None instead
of raising an exception" behaviour to the DBCursor set methods.
This will become the default behaviour in pybsddb 4.2.
* Fixed a typo in DBCursor.join_item method that made it crash instead
of returning a value. Obviously nobody uses it. Wrote a test case
for join and join_item.
