Ensure code quality with PHPStan

[nichtich]

As noted in #143, there should be continuous integration scripts. I added composer scripts to executed PHP lint, static code analysis with PHPStan, and unit tests localed in tests/ with PHPUnit. To fully enable it, there are a couple of errors detected by PHPStan (run composer analyse to get a full list):

• $dataElementCounts not defined in classes/Completeness.php
• access to undefined properties in classes/ControlFields.php
• undefined variable $db and access to undefined properties in classes/Data.php
• undefined method getRecordIssues() in classes/Issues.php

More warnings will come up when increasing --level of PHPStan in composer.json, but I think level 0 is enough to start with.

[nichtich]

Note to self: add a pre-push-hook so failing tests and analysis is never pushed.