automatically spawn additional download threads

[cvandeplas]

If for whatever reasons the download threads are not catching up with the new pasties arriving the queues will grow and end up eating lots of memory.

Add a new feature threads: auto that automatically manages the creation of additional download threads for that specific website.
The user can then choose to let pystemon decide what's good (auto) or hard-configure a number of threads per paste-site (like today)