ENH: Added mask for fixation cross

Created a transparency mask for "cross". Can be used for presenting a fixation cross in an easier manner.
Example use: fixation = pspy_vis.PatchStim(win=experiment_window, mask='cross', pos = [0, 0], size=1, sf=0)
Merge pull request #557 from suddhasourav/master

ENH: Added mask for fixation cross