Explicitly import lists:append/3

Without the import, SWI-Prolog occasionally produced errors like:

    PROLOG SYSTEM ERROR:  Thread 1 trapUndefined(): undefined:
    function_expansion:append/3

Explicitly importing the predicate solves the problem.  I assume this
has something to do with autoloading during term/goal expansion.  That
environment is somewhat fragile.
Add library(tap) tests

These small tests cover those situations described and modeled in the
documentation.  I want to be sure that our publicly advertised
examples work correctly.
Ignore gift-generated tar balls
Bump version