- Update Readme to include details about unit testing

- Add .gitignore file
- Add `/tests/` directory and `.gitignore` files to .gitattributes's export-ignore functionality

Refactor tests to more standardised format

- Add PHPUnit to composer

- Update readme to include instructions for installing composer packages and running unit tests through said package
- Use composer's autoloader to include mPDF during unit tests
- Update gitignore so the vendor directory is excluded