Implement Min/Max view functions.
Implement updateMin, updateMax and equivalent withKey functions.
Rewrite deleteMin, deleteMax in terms of updateExtremity.
Add a test for null

Adapted and simplified from a pull request by Ramses de Norre.
Fix deleteFindMin
Fix deleteFindMax
Small improvements to the test code.
Merge pull request #20 from bcdarwin/master

implement updateWithKey and rewrite delete to use this
Merge