Added a new function to convert SkyCoord to pixel coordinates

@mdboom @nden - would something like this be acceptable in astropy.wcs? I need it in various project and it will also make the implementation of https://github.com/astropy/astropy/pull/2210 easy.

If you think it would be ok, then I can go ahead and add some tests and a changelog entry.
The reason I didn't make this a method originally is that not all WCS are celestial so I wasn't sure about adding this kind of method that would be very specific to one type of WCS.