Add demos

[hoechenberger]

Should include some simulations and plotting.

[mmagnuski]

Hi Richard, since I have already created the script for testing the running-questplus-from-psychopy-coder bug, I could turn it into a short demo for questplus. For example: creating and plotting the ground truth weibull, then running questplus fror about 100 trials, plotting how the fitted weibull and posterior for the parameters change (a gif or static figure with for example values for trial 10, 50, 75 and 100) and showing the final comparison (recovery of the ground truth weibull function).
If you agree that this could be a nice addition then let me know how to proceed - where the file should be created and with what naming etc. :)