Cleanup procs

Maintain a flatfile containing the process identifiers of all active activity servers. Ensure that any "zombie" processes are killed whenever starting the top-level server. (This solution originally described in gh-118.)

@mzgoddard Mind taking a look at this?
If you can believe it, this inconsistency was intentional. I don't like using that feature of Mocha when I'm testing APIs that return a promise because if they *don't* return a promise (for some reason), you end up with a false positive. [Here's a silly example of this.](https://github.com/kadamwhite/wordpress-rest-api/pull/44)

What do you think about my recommendation there, instead (storing a reference to the promise object, checking its shape explicitly, and *then* `return`ing it to Mocha)?