add RGBColor readCurrentColor(BlinkLed led) to Blink

this method return the current color of the specified led (LED_1 or LED_2).

- add BlinkCommand, ancestor of ChangeColorCommand and ReadColorRequest
- sendCommand from BlinkDevice now take a BlinkCommand (instead of a ChangeColorCommand)
add readCurrentColor documentation
Merge pull request #6 from avernois/read_rgb

Read current rgb color