Switched Download Center to requests.
Fixed small and medium tests.
New SSL certificate: this one is valid for hostname 'localhost'.
Introduced 'requests' as a dependency.
Assorted small tweaks.
Nicer error message on invalid schema.
Removed some unnecessary assertions.
Minor tweaks.
Merge pull request #24 from Tinche/master

Merging switch of Download Center to requests to support redirection and other network features. Thanks to Tinche.