Make 'make tap' the default target and add a 'make phpunit' target to

Make 'make tap' the default target and add a 'make phpunit' target to
run things with phpunit.

phpunit as-is consumed up to 800MB of memory on PHP 5.2.12. Running it
also results in test failures for LanguageConverterTest.php which is
tripping over some setup code a previous test made.

The TAP target also runs in ~15 seconds on my machine, whereas I have
to kill phpunit after 8 minutes of running tests & eating my memory.