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.
Having it do that would be easier, but it might not always be the desired behaviour. I am just concerned that in general there is no real provision for extracting data in predictable fashion which preserved things like custom representations.

(as an aside, I think there should be a way to convert a representation to a `Quantity` object for piping into different code)