Add phpunit as a dependency in composer.json

In order to run the test suite, phpunit is required.
Rather than having different instructions for installing, we could put it in as a requirement in the composer.json file and have it installed automatically.

```bash
# Download composer
curl -s https://getcomposer.org/installer | php

# Install the dependencies
php composer.phar install --dev

# Run the vendored phpunit
./vendor/bin/phpunit --stderr --bootstrap tests/bootstrap.php tests/tests.php
```