Cleaning up description of GNU GPL in the index.html and changing the note in the gplv2 and gplv3 license pages

I made some minor changes to how the GNU GPL is described in the index.html page as well a change to the note on the GPLv2 and GPLv3 license pages stating that all source code files should state their copyright and licensing terms, which I think could make sense for all other licensing pages as well. 
I think the first sentence is actually clearer in the pull request than the original. The original says it requires you to distribute the code under the same terms but doesn't describe the terms. There's probably a better phrasing than "protects free code from being turned into proprietary software" but at least it says something.