GNUmakefile: More explicit success and fail message, hopefully.
GNUmakefile: Hopefully better fail message (and refactor it)
GNUmakefile: use $< instead of $^ in run/%

If the preparation of the run/file depends on something else than just
the script, cp will fail as $^ is being used.

This patch allows run/% to also depend on the initial configuration file
(in a later patch) without letting cp break.
GNUmakefile: ensure test/* --help don't fail because of exit 1

While some of the --help may exit with exit 0, some may do it with exit 1,
which would stop Make.
Remove 'welcome' challenge for an initial config

This is quite a change. There was a chicken-and-egg problem:

We wanted to automatically hint the user with the file name to use for
his challenges. During development, this was taken from the extension
chosen from welcome.* but then the name displayed for the 'welcome'
challenge was misleading because there was no welcome.* file by then.

So let's remove the 'welcome' challenge altogether and let all challenges
depend con a configuration file which, if doesn't exist, displays the
help text that was originally in the 'welcome' challenge.

In this initial configuration three directives will be stored: language,
mimetype and extension. Sample configurations will be provided for the
user under the config-samples directory. The user should only need to take
the chosen sample and copy it to the working directory.

The welcome message was moved to test/messages/welcome, which is now a
plain text file.

References to the welcome file have been updated.
GNUmakefile: remind the user of the filename for the next challenge

Because of this, the file name reminder in test/execute is not needed
anymore.
GNUmakefile: alert the user on lack of hashbang header

Lack of the hashbang header will allow the scripts to be run as Bash
scripts which may falsely pass. For example, in the 'hello' challenge,
if a user uses "echo hello" instead of "print 'hello'", it will pass if
the hashbang header is not present.
Refactor the completion message display. Tweaked it a bit too.
Include Python solutions for all the challenges