Add PDO repository (#2)

* Add PDO repository

* Make file storage path configurable

* Name username query property required

* Add special exception class

contracts/Exceptions/InvalidPathException.php

@@ -0,0 +1,21 @@
+<?php
+
+/*
+ * This file is part of GitHub Profile Views Counter.
+ *
+ * (c) Anton Komarev <anton@komarev.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace Contracts\Komarev\GitHubProfileViewsCounter;
+
+use InvalidArgumentException;
+
+final class InvalidPathException extends InvalidArgumentException implements
+    GitHubProfileViewsCounterThrowableInterface
+{
+}