Renames phpunit.xml to phpunit.xml.dist

Running phpunit will still work, but now users can create their own
phpunit.xml and override the distributed config.

phpunit.xml has been added to .gitignore, as has composer.lock and
vendor.
Removes vendor and composer.lock from .gitignore
Merge pull request #40 from jeremykendall/phpunit-config-change

Renames phpunit.xml to phpunit.xml.dist