Increased compatibilty with latexmk by providing warnings on missing files

The code comes from John Collins <collins@phys.psu.edu>, the author of latexmk.
The warnings apparently make latexmk aware of the missing files and then runs
pythontex.

Since 0.11 is released already, I took the liberty to created a new changelog
entry under a newer version.

I changed the .dtx file and ran pdflatex on .ins and the .dtx.