Implement an autoloader (#4140)

* Implement an autoloader

When cleaning up classes for psr2, things got a bit unwieldy, so I implemented a class autoloader.
I created a PSR-0 compliant LibreNMS directory and moved all classes there that made sense.
Implemented LibreNMS\ClassLoader which supports adding manual class mappings

This reduces the file includes needed and only loads classes when needed.

* Add teh autoloader to graph.php

* Add a small bit of docs
Fix incomplete class in includes/discovery/functions.inc.php

LibreNMS/Exceptions/FileExistsException.php

@@ -0,0 +1,32 @@
+<?php
+/**
+ * FileExistsException.php
+ *
+ * -Description-
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * @package    LibreNMS
+ * @link       http://librenms.org
+ * @copyright  2016 Tony Murray
+ * @author     Tony Murray <murraytony@gmail.com>
+ */
+
+namespace LibreNMS\Exceptions;
+
+
+class FileExistsException extends \Exception
+{
+
+}