Add PHP_CodeSniffer and fix errors

* Add PHP_CodeSniffer as a dev requirement
* Enforce PSR-2 checks during `composer test`
* Fix errors from PHP_CodeSniffer
Rename Wikimedia\Composer\Merge\Plugin to Wikimedia\Composer\MergePlugin
Add first unit test

Add a first unit test for the simple no merge conflict case with
a single additional file.