Copy the config.php file upon installation.

This allows us to know whether Flarum is already installed, so that
we can disable certain service providers when it isn't.

This should fix #67.

src/Core.php

@@ -0,0 +1,9 @@
+<?php namespace Flarum;
+
+class Core
+{
+    public static function isInstalled()
+    {
+        return file_exists(base_path('../config.php'));
+    }
+}