Handle POST to /data/fs/[path]

Each line of the body is submitted to MongoDB as a "save" (which is effectively update/insert).

Given the MongoDB model, this actually supports creating a new collection, saving new documents to an existing collection, and updating existing documents (see the README).

All lines are parsed before any data is saved, so nothing is changed if the input is bad. However, partial writes can occur if the MongoDB driver gives an error.

Also added a command `save [path] [object]` to the REPL, and made the `ls` and `cd` commands actually work, for the most part.
OK, on PUT: Be sure to delete the temp collection if it fails before the end.

On POST: I think `Process[Task, RenderedErrorJson]` is sufficient. `Unit` is not quite sufficient, as it's necessary to capture the malformed JSON if possible because users will otherwise have no idea which entries are malformed / were rejected. But with this basic signature we can implement failure criteria or other things on top.