Development Environment Flexibility and Continuous Integration Enhancements

Changed phpunit.xml to phpunit.xml.dist for portability and allowing for environment overrides. Enhanced the .gitignore for wider handling of diverse development environments. This should fix Celc/CIUnit#11
Only add comments to provide more context. Since it's a `.gitignore` file, we already know it's ignoring something. It would also strictly speaking ignore a file called `vendor`.