Decode tile off the main thread

[kylebarron]

Closes #31

Change list

• Add ThreadPool Python class that wraps a rayon threadpool.
• Add Tile Python class that wraps an underlying tiff tile
• Add rayon and tokio_rayon as dependencies to help manage cpu-bound tasks and the interface to the async executor.

This ensures that the executor isn't blocked during decompression.

We haven't piped through the interface to actually fetch tiles yet, but this is mergeable in isolation.