Tools: Add python script to run one simulation several times

multiRun.py - run an exported cooja simulation several times

Cooja can export a complete simulation setup to a jar file. This can be executed independently from the contiki / cooja environment. This python script allows to execute one simulation file several times with different seeds. This is useful for example for montecarlo evaluations of a simulation setup.

I introduced this tool at contiki-developers and got a lot of feedback. I think this tools would be useful for anyone who needs to execute a cooja simulation several times.