BF: bits.py should avoid importing pyglet.gl until needed to reduce wx probs

wxPython still has a problem using Choice entries if pyglet.gl has been imported
BF: allow CRS Bits# to correct gamma when generating LUT

We'll call this the 'software' gamma correction because it's controlled
by PsychoPy at run-time. The 'hardware' method will use the gamma LUT
stored on the Bits# itself (not yet implemented)
Merge branch 'master' of github.com:psychopy/psychopy