Add strategy pattern for alien behaviour

src/aliensimulator/Alien.java

src/aliensimulator/AlienMars.java

src/aliensimulator/AlienMond.java

src/aliensimulator/AlienSimulator.java

src/aliensimulator/AlienVenus.java

src/aliensimulator/ErdeBesuchverhalten.java

src/aliensimulator/ErdeFeindlichBesuchen.java

src/aliensimulator/ErdeFreundlichBesuchen.java