Added a build.gradle

Libgdx's is now using gradle as its dependency management.

I thought that using gradle for gdx-artemis was fit, this way it can always pull the latest libgdx version by just adjusting the build.gradle file (one line).

Another advantage to this is the fact that gradle is able to set up environments for eclipse or idea (i think netbeans as well).

Please let me know what you think