Inconsistent licensing headers

As pointed out by @KBar in the mailing list, this plugin has two COPYING files and headers in source files are inconsistent, there are mentions to GPL and LGPL.

Unfortunately it's hard to tell what was the original author's intention, because it's been like this since the initial commit. My guess is that he meant dual licensing, but I have no idea if we can change it GPL, LGPL or clarify that it's dual licensed.