Move WCS calculations to astropy

[wschoenell]

We currently calculate when exposing, on the camera saveImage process:

chimera/src/chimera/instruments/camera.py

Lines 172 to 177 in e15ce55

	img += [("CRPIX1", CRPIX1, "coordinate system reference pixel"),
	("CRPIX2", CRPIX2, "coordinate system reference pixel"),
	("CD1_1", scale_x, "transformation matrix element (1,1)"),
	("CD1_2", 0.0, "transformation matrix element (1,2)"),
	("CD2_1", 0.0, "transformation matrix element (2,1)"),
	("CD2_2", scale_y, "transformation matrix element (2,2)")]

For this few parameters it is ok this approach, but since we are getting more complex I suggest to move this wcs stuff to the Image object using the astropy wcs package:

http://astropy.readthedocs.org/en/latest/wcs/#building-a-wcs-structure-programmatically