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 argument that not all WCS are celestial is a good one against.  However, I don't think we should have `world2pix` support SkyCoord if there isn't something equally convenient to get SkyCoord out of `pix2world`.  (Maybe if you pass in a quantity whose unit is "pixel"?  Or maybe that's crazy.)

I agree WCS is already rather long, and having the implementation here is fine.  So maybe it's fine to merge this more-or-less as-is and think about the more OO interface, if any, later.