Add extract-native-libraries-from-jars? option

This searches uberjars on the classpath for <jar>/native/linux/*/*.so native android libraries and includes those libraries in the build, according to the file layout at https://github.com/swannodette/native-deps.

Should not affect existing native-libs handling at all but offers an alternative to package native binaries with jars for android.

Current issues: 
* It's a bit slow - adds half a second on my current classpath. Could do with some way of realising it's already checked jars and only refreshing on a change.
* Copying happens on every build, again, could do with a method to determine that the file is already extracted and skip
* Probably not in the right place/namespace, should happen a little earlier in the process and simply add its directory to the normal native-libraries-paths vector.

If you want to suggest/review any changes I'd be happy to tweak a little more.