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.