Intentionally cripple Box's .get method for Lift 3.

Box.get now returns Nothing, which should break anyone
attempting to use its return type for anything, including tests.

Also fix a Box test that relied on .get.
Drop open_! and openTheBox, which are deprecated.
Fix several lift-util tests that used Box.get.
Merge pull request #1530 from lift/lift-3-box-get-out

Disables Box.get in Lift 3. It also removes open_! and openTheBox, which were deprecated in Lift 2.4.