Requests support

Hi,

as discussed before. Note that I've created a new SSL certificate because requests verifies the host name by default (otherwise a whole aspect of HTTPS is undermined). The certificate included is valid for 'localhost'.

I've also included tests for redirection (forgot to mention in the commit message).

These changesets pass pep8, small and medium tests on my machine.
Do you think we should catch that error in the DownloadCenter and transform it to a more user-friendly message? This message is potentially presented to the end-user (let's say there is a wrong redirect on a website to apt:// for instance or some android prefix…). What do you think?