[tests] Use Predis\Autoload from include_path when not in repository.

This is useful only for running the test suite after installing the
PEAR package, see #126 for reference.
[bin] Add helper script to automate PEAR package creation.

Note that this script takes care of customizing `phpunit.xml.dist` so
that tests can be run as soon as the PEAR package has been installed
as requested in #126.
[bin] Add missing executable in the repository for new script.