Eliminate Guava depend; create Resource helper.

The Guava dependency was a ticking time bomb waiting to screw any project that
happened to use Guava and which, for whatever reason, ended up having its real
Guava depend "further from the root POM" than the playn-java's (ancient) Guava
depend.

We were only using Guava for a couple of tidy things, which were easy to
reimplement, so let's just drop it and save future people the effort of
annoying Maven dependency debugging.

This also introduces JavaAssets.Resource which does the various asset decoding
more efficiently when we know we're operating on files. Image decoding, audio
decoding and even font loading all have more efficient paths (in the JDK) when
you can provide a File instead of an InputStream. More wins.