Add an example of calling Mercury libraries from Java.

In particular, demonstrate how to invoke any module-local finalisers and
retrieve the exit status.

samples/java_interface/standalone_java/Makefile:
samples/java_interface/standalone_java/JavaMain.java:
samples/java_interface/standalone_java/mercury_lib.m:
	As above.

samples/java_interface/README:
	Describe the new directory.