tumblr_backup: use a timeout for HTTP requests (fixes #28)

Relaxing the exception handler to IOError also catches socket errors.
tumblr_backup: provide a way to limit the pool (fixes #27)

The workers are a lot slower than what feeds the queue. On a decent
sized blog of dozens of thousand of posts, the queue will use several
hundred MB of memory.

Limiting the queue to 1000 will block the process of getting posts from
Tumblr.