Fix Box->Option implicit conversion spec.

We were testing Empty to ensure isDefined returned false, but
isDefined hasn’t triggered an implicit conversion to Option in a
while. We now test the implicit conversion by verifying that
orElse exists and behaves as expected on Options.
Fix a reference to open_! in the Box specs.
Define a get method on Box and immediately deprecate it.

We do this so that get no longer triggers an implicit conversion
to Option, and triggers a deprecation warning. In the future,
get will remain defined but will throw an exception and be
defined as returning an internal vestigial type to break
compilation.

Those who still want to use .get can call .toOption.get.
Merge pull request #1523 from lift/1523-gettin-boxed-in

Define a Box.get that is deprecated to discourage people from using it since it's not the "correct" way to open a box.