Experimental hacks to use JDK8. Not backward compatible with JDK6.

FOR REVIEW ONLY. DO NOT MERGE TO MASTER.

These changes seem to be sufficient to make JDK8 work.
This module's unit tests pass, and I have evidence that gremlin-node
now accepts references to classes that were compiled for Java 8.

We now have to figure out how to maintain backward compatibility for JDK6.
Perhaps we can simply detect which JDK is installed in JAVA_HOME and
compile for it?

This is tested only on my Mac with OS 10.9.5, and with jdk1.8.0_25.
The commit comment says "mac only", but the only iffy thing I did was change
compile-java-code.sh to use ${JAVA_HOME} instead of /opt/jdk1.6.0_45,
which I think should improve compatibility across platforms.