Sleep one second when receiving HTTP 429 error

One of our mirrors has started rate-limiting us. It seems this mirror is selected for me 100% of the time, and tarball conversion fails 100% of the time. Ouch. We can avoid this by sleeping for one second and retrying when it occurs.

Note this is called on a thread, so it theoretically only affects the thread that actaully received the 429 response. This means that other threads are likely to hit the same error, but that's OK.

Fixes #23 (closed)