Adjusting the Guess exercise.

This makes the Guess exercise suitable as a 3rd exercise.
Using latest codeclub.py code.
Rewording exercise descriptions.
Rename.
README amendments.
Improving links from front page.
Tweaks & syntax fixes.
codeclub.py: Fixing bugs in directory search.
Latest Alien problem code.
Minor tweaks to Py01 docs.
Re-adding code-problem which went awol in Py01.