Update class_xml.php

Add function/s:

parse() -> Initiates Parser, easier to use this.
getXMLElement -> If you have complex XML like:
```
<?xml version="1.0" encoding="utf-8"?>
<info>
         <data>
 		<name>INS Test Addon</name>
 		<author>AskAmn</author>
 		<description>Test build for testing INS Applications/Hooks Pages</description>
 	</data>
</info>
```
MyBB's parser will surely give you something that you wont like :)

a print_r after parsing gives:
```
Array
(
    [info] => Array
        (
            [data] => Array
                (
                    [name] => INS Test Addon
                    [author] => AskAmn
                    [description] => Test build for testing INS Applications/Hooks Pages
                )
        )
)
```
That also needs a lot of work to get to 'name', 'author', etc.
But now, you can use: (Example given in Comment in code.)
$xml->getXMLElement($array); to get to 'name', 'author', etc easily.
This gives us:

```
Array
(
    [name] => INS Test Addon
    [author] => AskAmn
    [description] => Test build for testing INS Applications/Hooks Pages
    [last] => 1
)
```

// Wow, that's so cool! :P

Also changed the constructor function so that we can directly open an XML file.
Also added a var $init to constructor for lazy dudes to automatically feed the self::parsedXML var.
Also added $parsedXML for easy access to parsed XML.