finally allowing a standalone build, using a provided JME3 for compiling against.
Do not use the provided JME3 for more than building the project, it is Excpected to use a own full JME3 build.

Later when jme3 has maven artifacts, the build will change to use that instead of the provided one.
Prepared, to allow publishing to a maven repository

Signed-off-by: Kai Börnert <kai-boernert@visiongamestudios.de>