[wip] Spawning pool

Time to shave off that final 2-second delay. This PR implements #32 by pre-launching a configured *(by which I mean hardcoded to "3")* set of containers and handing them out to incoming requests. When a container is culled for inactivity, it's scrapped and a new one is launched and added to the pool in its place.

If you hit `orchestrate.py` with a path already set (using a stale link, for example, or an appear.in-style link like #67 implements), you have a personal container created and linked as it happens now.

/cc @rgbkrk so he can see just how terrible this is right now, and suggest embetterment techniques :wink:
> This makes me wonder about restarts. If I set my pool size to 1000 and restart 3 times, do I now have 3000 containers?

Yah. One potential workaround for that would be to scan the proxy setup at launch and use proxy route metadata to reconstruct the state of the spawner. Of course, that assumes that the proxy and Docker are in sync...