Add builtin modpack 

Add the ability to use a modpack.xml in the resources directory to add a 'built in' modpack
without having to re-compile the launcher.

Idea being that some random (like me) can download the launcher, extract it with jar -xf (or open it in winrar), add a file in resources called modpack.xml with contents like below

```xml
<modpack>
	<configuration>
		<id>packid</id>
		<name>PackName</name>
		<appDir>packdir</appDir>
		<updateURL>http://url.to/update.xml</updateURL>
	</configuration>
	<!-- For future expansion perhaps?
	<servers>
	</servers>
	-->
</modpack>
```

for distribution to the players of a server.

Without having to actually re-compile the launcher