Add Apache 2.0 license files

[tbouffard]

Currently, except in the source code, there is no license information. For instance

/**
 * $Id: Connection.java,v 1.2 2013/04/08 07:14:16 mate Exp $
 * Copyright (c) 2013, JGraph
 */

This makes it hard for svg2xml contributors and users to know how they are allowed to use, distribute and modify it.

I am not sure that the Apache 2.0 is the right license for this repository, I used the one which is the most common in repositories of the jgraph organization. You may want to use another license type, like in jgraphx, see https://github.com/jgraph/jgraphx/blob/v4.1.1/license.txt

cc @alderg @egrimate @davidjgraph

[davidjgraph]

Added directly, thanks.