Initial support for reading data files out of zip files.  One thing that still bugs me a little is that if one only uses get_pkg_data_contents with a file in a zip file it's not necessary to first extract that file to a temporary location--we can just read it directly out of the file.  But after some experimenting I found it not worth the extra code complexity for now to treat that case specially.
Cleaned up the Table.show_in_browser code just a bit, and fixed it to use the get_pkg_data_ API
Removed unused function