#359 Upgrading to FreeMarker 2.3.21; Updating license.txt

@agudian, can you check out this one, some of your Maven-foo is needed :)

Turns out the latest FreeMarker also uses the Apache license which is nice as everything uses the same license now. I've updated the license.txt file. Here comes the issue though: The shading pulls in LICENSE.txt and NOTICE.txt from FreeMarker into META-INF of the JAR. The latter is fine, but the former should be replaced by our own license.txt (which mentions the FreeMarker copyright holders).

Could you give this a try, I didn't manage to do so :/ Ideally, our license.txt should be in all our JARs, and optionally we could put the FreeMarker one to META-INF/FreeMarker for reference. Thx!