Make modules know their version

Feast your eyes: now `ModuleInfo` has a `getVersion()` that is robust:

* For commands, it is the `Implementation-Version` in the manifest of
  the JAR containing the command.
* For scripts, it is a timestamp (for human readability and sortability)
  followed by the SHA1 hash of the script contents (to guarantee version
  uniqueness).

This branch also adds a DigestUtils with many useful methods for SHA1
computation and related things.

And things are even unit tested. All for the low, low price of a quick
code review.