Integration with SkyCoords

[mileslucas]

I think in general we could have better coordination between WCS.jl and SkyCoords.jl. For example-

wcs = WCS(...)
px = world_to_pix(wcs, ICRSCoord("04:05:23.33", "22:02:56.69"))
50, 39

and perhaps return

world = pix_to_world{ICRSCoord}(wcs, 50, 39)
ICRSCoord("04:05:23.33", "22:02:56.69")

I don't think this would be difficult, but we should put some foresight into this as we start to entangle the dependencies.