Install phpunit.phar as phpunit into PHPBREW_BIN via "phpbrew install-phpunit"

Hi, all.

I fixed the issue #240. By this pull request, we'll install phpunit via phpunit.phar not pear when you execute `phpbrew install-phpunit`.
ok no problem, I'll add it to the patch release after doing some more tests. People can always build from develop in case they really need it anyway.