Use Cmdliner for command-line parsing

Side-effects:
 - more uniform handling of --help and --version, automatic generation
   of Unix manpages
 - slight change in syntax (see demo.sh)
 - sub-commands of "election" (Tool_election) are now direct commands,
   "election" itself disappears
 - in Tool_election, concatenate with --dir option (if any) instead of
   calling chdir
Update installation documentation
Stricter syntax for Tool_election commands
Add a rule for js_of_ocaml