pep cleanups of tutorial code, plus some minor changes (dt)

I just couldn't resist editing the tutorial's code when i read it, changes are:
- kv code indented 4 spaces, not 1tab
- the Basic().run() line was always indented 3 spaces, not 4, fixed that
- lots of trailing spaces on lines removed
- two lines between each class on python code
- i put two lines between rules on kv too, you can disagree with thi change
- commented or deleted unneeded imports
- added comment on needed import for kv
- changed the update clock time to 0 (update each frame) instead of the 1.0/60.0, i find it clearer for the same effect, you may disagree
- other pep fixes (spaces around symbols, indentation in parens, braces and others)