[Improvement] Add transistion time

[neoteknic]

Add transition time to setRGB($r,$g,$b,$transistiontime=null) and other method in the light class.
I will send a pull request soon.

[sqmk]

This should belong in the SetLightState command, and not applied to every Phue/Light state changing method.

If you wanted to, you could implement the model design that I use for managing lights, sensors, groups from https://github.com/sqmk/huejay. With that library, you can change various properties on each individual light, and then save each light, with all updated attributes being sent to the bridge.