Feature/issue 31 life cycle api

[rohan-deshpande]

Added

• The core/System class now has its own emit method which takes an options argument where the user can provide onStart, onUpdate and onEnd methods. These are wired into all emitters via the new experimental_emit method on the Emitter class
• New event to signal an emitter coming to an end called EMITTER_DEAD
• Optional argument to fromJSONAsync method to determine if the system shouldAutoEmit or simply set up its totalEmitTimes and life - this allows users to use the new System.emit method and provide their own life cycle methods
• isEmitting boolean property to emitters to get/set if they are emitting or not
• LifeCycleApi website example
• TESTS!

Changed

• An emitter now will no longer update if it is not emitting particles
• The EMITTER_DEAD event will fire when the emitter is destroyed and there are no particles left