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.
perhaps the ``origin`` keyword should be available in the convinience function?  That is, change the signature of this function to ``(coords, wcs, wcsorigin=0)``, and then to the world2pix call as ``wcs.wcs_world2pix(lon, lat, origin)``