Allow download of compressed data

[deeplook]

Given that data files to be downloaded from the store can become pretty large (tens of MB and more) it would be a very nice feature to be able to download them in a zipped version (compressed on the server) as well. So, in addition to

http://webstore.thedatahub.org/okfn/foo
http://webstore.thedatahub.org/okfn/foo.json
http://webstore.thedatahub.org/okfn/foo.csv

one might think of URLs like these:

http://webstore.thedatahub.org/okfn/foo.gz
http://webstore.thedatahub.org/okfn/foo.json.gz
http://webstore.thedatahub.org/okfn/foo.csv.gz

where the compression is done using the standard zlib library.