Cleanup: Convert tabs to spaces. Remove trailing spaces.

Convert tabs to python standard (four spaces) in all files. Also remove trailing spaces. Indentation should be the same over all files now. 
I noticed in some places there were tabs before comments - there should be two spaces before and one space after the hash character - now there are four spaces instead of the tabs but IMHO this is only an aesthetic problem.

Hope you can agree to this as it looks like such a huge change. But if I got it right nothing of relevance for the Python interpreter has changed. The indentation stuff just kept causing me trouble.