lib/java: add services to support Java strings

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
tests: adds a test for java strings

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/jvm: change a C function to a Nit extern method

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/jvm: rename JObject class to JavaObject

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/java: intro JavaObject::new/release_global_ref

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/java: improve documentation

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>