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.
Ok, I've added a TODO to remind us to re-assess this. The OO approach might change once the generalized WCS is implemented in any case, so maybe that's a good argument for delaying. The functions here are already useful as-is for a number of affiliated packages.