Added current Java version

Before starting on the official version of this interpreter, I am
writing a version in Java partly as a proof od concept, and partly
to become more familiar with the design of a program of this sort.
The source code for the current state of that version (which is far
from complete) is now located under java/. Feel free to build and
play with it, but be warned that it is not well documented, and the
included REPL loop only works for the core language (desugaring has
yet to reach a usable state).