Major changes!!!

### plot.abscissae ###
- addition of legends in the plots
- legends according if h(x) and f(x) are plotted or not
- redefine the limit of the graphics, because it was taking
enviromental variables for l_f and u_f

### ars ###
- addition of legends in the plots
- accepted and rejected points with colors according
- include a pdf (optional) with the histogram of the generated sample