Added __isnull column lookup for Django-style NULL and NOT NULL value checking
Thanks mane!  Looks great
 No problem!

I realize this can be accomplished with a Q object, but it's arguably harder to read queries using them. This also is available in the standard Django API, so it may not hurt to have more feature parity on this level. 

Also, added some tests for "__isnull" and for use in conjunction with Q object (no idea why one would ever do this though).
