Switch to using official kubernetes client library

[yuvipanda]

https://github.com/kubernetes-incubator/client-python didn't exist when we started this spawner. It exists now, and we should use it.

It has async support of sorts, but it merely just spawns a new thread per request. I'm not sure if this is a valid way to do this, given we make a ton of requests. We've already had problems with this (see #30) We have a few options:

1. Determine that spawning a ton of threads is ok, and works fine. Test that we have connection re-use and things like that
2. Implement the async-ness ourselves, with a threadpool or something like that
3. Rewrite how we use the client - maintain a global, updated copy of all pods in namespace (with a list + watch), and use this copy to implement the polling operation. Do (1) or (2) for spawning and deletion.

I'd highly prefer (3) - that's what all the kubernetes components use, and it's also super efficient. It'll allow us to scale to a much higher number of users than we can today.

[yuvipanda]

Done