Update of README to represent implementation and Added setLightstate for Groups of Lights

I just found a small inconsistence in README. The input parameters for setting lightState with hsl(hue, saturation, brightPercent) is not described like implemented. 

hue is implemented as value between 0 and 359 (degrees) and
saturation is now also implemented as value between 0 and 100 (percent) like brightPercent.

Re-calculation to expected values by hue API is done by _getHSLState in lightstate.js.

Furthermore I added the function to set lightStates for a group of lights via one request using the Group API (http://developers.meethue.com/2_groupsapi.html). Added an additionally parameter to setLightState if it is a lightState change for a group. Then it will use the different REST endpoint with same request payload. Otherwise (when no group) it will use the standard endpoint for a single change of one lightState.