BlinkenWall: double frame buffering and new functions

 * double framebuffering reduces flickering due to
   no more needed TLC.clear() calls and reduced
   TLC updates. As a side effect, one can overdraw
   previous animations, if the last clear() call is ommited

 * added two new functions drawLine() and drawRect()

 * drawLine: uses Bresenham's line algorithm to draw lines

 * drawRect: draw filled rectangle (border color and fill
   color  are different parameter and may differ)
BlinkenWall: added examples

 * Added 2 simple Examples