Rename and clean-up modules `Target` and `Artefact`
Prevasives.Time: add type `t`
Implement “direct” commands on localhost
test: add direct commands (successful and failing)
build: put `.mli` files before `.ml` ones

It seems that `ocp-build` does not figure them out by itself.
Add module Ketrew_unix_process

This module was (almost) in EFMP and should move to `Pvem_lwt_unix` one day.
Add (basic) support for “SSH hosts”

We can now run basic commands over SSH.
test: add a successful direct command over SSH

The test looks for the environment variable `ketrew_test_ssh`; if not
defined it will use `"localhost"`, for the test to succeed it should be
a passwordless SSH host (the only command run on it is for now `ls /`).
Run the test like:

    ketrew_test_ssh=<Host> _obuild/ketrew-test/ketrew-test.asm [-no-color]

where `<Host>` is often an entry in your `.ssh/config` file.
doc: update the `README.md` file
Clean-up code
lib: implement direct commands that create files
test: add a direct command that runs over SSH
all: rename Artefact → Artifact (US spelling)
all: fix module hierarchy for artifacts/data
lib: check dependencies before stating a target

This is not really tested and requires some clean-up.