Semaphore for lecture speed

[nikolasibalic]

Adds new option presentationSemaphore when creating presentation

p = Presentation(
    ...
    presentationSemaphore=True, 
)

that adds semaphore for the audience (red, yellow and green) allowing each member of audience to indicate if the current lecture speed is ok, or too slow/too fast for the covered material.
The lecturer sees histogram of responses.