Added setLightstate for Groups of Lights via Endpoint /groups/<id>/action

extended apiPaths with groups/<id>/action and implemented a switch in
api.js setLightstate if additional parameter group is set with true. If
not just ignore it and use endpoint for single set of lightState.
changed readme for function hsl() where parameter in documentation are not described like implemented

Documentation of function hsl(hue, saturation, brightPercent) updated,
because implementation has changed. Input value for hue are degrees
between 0 and 359 (°). Input value for saturation is percent between 0
and 100 (%). Re-calculation for these inputs will be done by
_getHSLState in lightstate.js
- Adding a test for registration, its location means it will not be included with the other tests
- Extracting combine method to utils module so it can be reused.
- New Object for defining a 'schedule' for the Hue Bridge
- Expanding the ApiError to support strings or other error objects in the constructor
- Updating default test values so that tests will work for me
- Adding improved support for Groups and Schedules to the API
- Adding the ability to update an existing schedule and bringing the README up to date with documentation to cover all the updates to the API.
- Added SBejga as a contributor and bumping up the version number of the API to 0.1.3