Use astropy coordinates everywhere

Now that coordinates in astropy has a stable API, I think it makes sense to go through `astroquery` and make sure that *every* interface that uses coordinates as an input accepts an astropy `SkyCoord` (or frame class), and deals with converting to the appropriate frame.  This may already at least partly work, but it looks to me from the docs that some of the interfaces require coordinate strings but don't accept astropy coordinate objects (e.g. http://astroquery.readthedocs.org/en/latest/irsa/irsa_dust.html).

Similarly, when coordinates are returned, they should be `SkyCoord` objects.

Even better would be to go through the examples and start using `SkyCoord` anywhere that it makes sense.

cc  @keflavich