EHN: new coord system working with ElementArrayStim

Moved the unit conversion function to tools module

Also fixed the orientation to work in the same direction as GratingStim. This
had been fixed with a hack before to alter the orientation during setOris()
but should have been fixed in the rendering routines instead.
EHN: new coord system working with ImageStim

also corrected buglet in Image demo
ENH: new coords system now working for text position

updated test images for text test suite
ENH: more efficient accessing of the posPix

We should only access posPix once because it's using a python property
which is 4 times slower to access than a normal variable.
ENH: MovieStim now supports new coords system