Follow-up to 359: handling of LICENSE.txt after freemarker-update

* I've renamed our license.txt to LICENSE.txt, following the naming conventions out there
* our LICENSE.txt will now be included in all distributed jars, within META-INF (for core, core-jdk8, and processor)
* the freemarker LICENSE.txt and NOTICE.txt are now retained in META-INF/freemarker/META-INF/*.txt (pardon the extra META-INF directory down below, I wasn't able to get around it easily...)

This PR also includes the (rebased) commit from PR #360
Interesting, that's what I had tried as well, but for some reason the file wasn't picked up.