Makefile: adapt _install to also install modules
UNIX/etc/neubot: add the api and users configuration files
Makefile: make sure that we run install with umask 0022
Makefile: sort .PHONY targets
Makefile: better uninstall

Teach the uninstall target to attempt to remove all the directories
in which we install files.

We use `rmdir $DIR` to attempt to remove a directory, therefore,
if there are other files inside the directory, the directory is not
removed.

Tested by adding [ and ls inside $BINDIR and by checking that $BINDIR
is not removed by `make uninstall`.
Move bin/neubot to UNIX/bin/neubot

Now bin/neubot is the script to run neubot under UNIX, therefore
move it below the UNIX directory to avoid surprising people who
browses the tree.
UNIX/bin/neubot: adapt NEUBOT_HOME to new directory layout

With plugins we install $(DATADIR)/neubot/neubot (which contains
the core) and $(DATADIR)/neubot/mod_* (a directory for each plugin).

Therefore update NEUBOT_HOME to be $(DATADIR)/neubot.
Makefile: tweak the uninstall target

1. use `-mindepth 1` because there is no point in trying to
   delete the / directory;

2. we don't need to `rmdir {} \;`, therefore don't do it.
Merge branch 'feature_update_Makefile'