Add pass-by-value API server

[kycutler]

Resolves #549

Also adds --num-processes argument for all server commands (Unix only).

Doesn't currently contain tests, though I've done a fair bit of testing on my own -- would appreciate any feedback for how to make automated tests work since pytest-tornado only seems to support one entry point.

I'm also open to other suggestions for what to name the new entry point -- I know "api" is a little vague.