add test describing skip behavior

For completeness, this test guarantees that if skip is called in
a "before" hook, that the actual action will be skipped as well.
print stack traces on test errors

This includes a line for our support fixture that always enables
stack traces in chai when an error occurs.
milestone overloads should stack on top of existing behavior

Currently if you defined a milestone for a before or after hook
it worked just fine since there was no existing function to call.
If, on the other hand, you wanted to add a milestone to a
pre-existing action, then your milestone would be called after the
existing function. This corrects that behavior.