handle PUT/POST/DELETE to /data/fs/[path] (Fixes #140)

Replacing the earlier PR (#152)
Note this accepts all the content as a giant string, so there are hard limitations on how large this can be. I believe you can obtain an `InputStream` from the request, representing the content, and probably *scalaz-stream* has a method to convert an `InputStream` to a `Process`, or we could do that manually, emitting one `String` per line of input.

May not have time to get this in before the beta, but it may be worth creating a ticket for since this will explode on larger inputs, and isn't streaming even though the `FileSystem` is.