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.
Do you think it makes sense to leave the main code here anyway and just call it from a method in ``WCS``? (the reason I ask is because the ``WCS`` class is already pretty long). Whatever you think is best :)