Implement recordPlaceAction and queryPlaceAction, using simple in-memory storage

[rfk]

In order to expose an API on different platforms, we need to have an API to expose :-)

In [1] @mhammond proposed recordPlaceAction and queryPlaceAction so they seem like as good a place as any to start. We don't need to bikeshed the API just yet, because we're more interested in the technical details of exposing it for this first prototype.

So, let's implement a basic memory-backed version of recordPlaceAction and queryPlaceAction that can associate an action to a particular URL at a particular time, and then query out the URLs on which a given action was performed, most recent first.

[1] https://docs.google.com/document/d/1g3OzUc38V-t4EXkO9MUOJ4_TmCqSsz9IJPGh0fCcIgg/edit#

[rfk]

@thomcc volunteered to take an initial stab at this in the meeting just now.

[rnewman]

@fluffyemily has a to-do list app that's a pretty good reference for this.