added juggler handled by automaticJugglerManagement

This let the user decides which juggler should be managed by the automaticJugglerManagement.

FFParticleSystem.defaultJuggler helps to define juggler to use for all
new instances (default value : Starling.juggler)

Juggler to use can be also manually set for each individual particle system.
Merge pull request #2 from aureliendc/add_user_juggler

Added juggler handled by automaticJugglerManagement

Thanks. :)