Implement Spitzer Heritage Archive query

I noticed that #45 mentioned querying the SHA through the SIAP protocol that they have setup. I've implemented the query to return a astropy.table.Table instance with the right dtypes using the requests library.

I'm not sure if this is redundant with the IRSA sub-module, but the SHA is mostly for querying meta-data and download links for image files, so it seems like it could be of use.