#31: added requiredUnless

A (trivial) implementation of requiredUnless. I did not address potential conflicts of making an option "required if" and "required unless" at the same time, except for mentioning it in the docs.

I also fixed a small bug I came across, where other dependents were not added to the map of conditionally required options, and added a test case in RequiredIfAnyTest.java. Hope I got this right.