Added ability to stage hypnograms.

Select the 'tent' icon from the toolbar, then stage by pressing the
numeric key associated with the stage you want to score (0 = [W]ake,
1,2,3,4, 5 = [R]EM, 7 = [U]nknown).  When finished, export your saved
.STA file by clicking on the menubar's file, export, hypnogram to .STA.