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.
Using ``data.lon`` shouldn't be hacky in the sense that this is the true underlying representation. In your view, should creating a new spherical representation preserve the Longitude/Latitude classes/settings? If so, it might make sense to open a new issue for this to discuss it.