Switched Download Center to requests.

How does this look for a first draft? I can successfully install android-studio with these changes (and my Eclipse Luna, but that will be another pull request).

There are failing tests, the first ones are about

<pre>
self.assertTrue(self.launcher_exists_and_is_pinned(self.desktop_filename))
AssertionError: False is not true
</pre>

which doesn't seem directly relevant (the launcher icons are created in the tests and normal usage). The tests take a very long time so my computer went to sleep after the first dozen. 
Maybe change to something along the line: "Can't download ({}): {}". This will wrap under a logger.error automatically on the _one_done() callback.
(for reference:
        if future.exception():
            logger.error("{} couldn't finish download: {}".format(future.tag_url, future.exception()))
)