Clean up distributed license files

Noticed that the gpl.txt and artistic.txt files were still being distributed long after the switch to boost.

There are some remaining files that are still dual gpl/artistic licensed, so the first commit is re-licensing them as boost - so only @WalterBright or the original authors (@rainers @dheld) can sign these off.

The second commit is removing the gpl.txt and artistic.txt licenses, updating the readme.txt, and adding a new boostlicense.txt file which is included in the ``install-copy`` make rule, which I assume is used for creating release tarballs/zips.