[App Inventor Open Source Development] Support for Java 8

I had tracked down the incompatibilities with Java 8 to guava, and
from there to Objectify, so I had a quick look and test with Jeff's
latest magical updates for GCS and voilà!

All libraries that needed updating have been updated, and I am running
my local server compiled with Oracle's Java 8 on mac. Will test on
Linux when I get a chance, but I'd appreciate some Windows backup. If
you guys have a chance, please report back.

Excellent!

Maybe someone with Windows and Linux can verify, and then we can update the developer docs (and our systems).



Tested on my machine. 
OpenSUSE Linux 13.2 64bit with JDK 8u25.
Successfully build and run on local server.



I'm only using the new guava on the server side. The old version is still used on the phone, in particular with the Fusion Tables and Texting components. So they may fail when compiled with Java 8. Obviously some testing is in order.

I do intend to update the libraries used on the phone (aka in the components) but that will require significant testing, and I didn't have time for that in our current "crises"mode.