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. 
Do you know what is raised if the protocol isn't supported? (see as well the failing tests), would be nice to try this (see my other comment on the small test failing).