Add jar ignore

see https://github.com/mspsim/mspsim/pull/31 This should also keep the submodule in Cooja clean.
Maybe I'm missing something, but IMO it would be safer if this is path-qualified, as in ```/*.jar``` (alternative: ```mspsim.jar```) so that only mspsim build artifacts are ignored, but _not_ the jars in ```lib/```. This would avoid mistakes when someone forgets to force updates to the libs (or more likely does not realize they are ignored).