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.
Yeah, let's treat POST as append (i.e. bulk API for insert, in the case of MongoDB), and then PUT as basically "delete + append", so we can probably use the same bulk API for MongoDB (albeit it won't be atomic, but then I'm not sure if there's a way around that in MongoDB).