Correct semantics of quickcheck properties
Allow just_value/2 in both modes

The property wrap_unwrap/1 describes how I imagined just_value/2
working.  The predicate's original implementation failed with a
domain_error when the first argument was a variable.

quickcheck FTW!
Correctly describe list_round_trip/1 property

The cases "same head" and "both empty" are mutually exclusive.  Make
sure the property makes that distinction.
Relate maplist/3 and map_maybe/3

This property is a little weak since it only describes the list's head
element, but we still want to be sure that it holds.
Release v0.0.2