Move activity update code to jarabe.model

In preparation for implementing automatic/background activity updates,
move the code from an extension-specific model into jarabe.

The main update code is now in jarabe.model.update and the downloader
has been made generic and moved to jarabe.util.downloader. It now
downloads files into the temporary 'data' directory which is cleaned up
on every Sugar launch, avoiding leaving stale files around in failure
cases.