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.
The problem with other Java implementations is that they will be mutable and not respect our file system model, and probably be less type safe than what we have now.

That said, I do like the idea of making relative and absolute paths different types. Then we can enhance signatures like ++ so that the rhs is constrained to be a relative path, and otherwise add additional type safety in lots of other places.

We might be able to solve at the same time the question of "what is a relative path relative to".