Send error message if flake8 executable not found
Speed: variables calculating indent or moves use `regexp-opt'

unused function `py-outdent-p' removed
`py-compute-indentation' faster

When checking if still at original line, reading
line-end-position should be faster than counting lines
New boolean py-debug-p, default is nil

When non-nil, keep resp. store information useful for
debugging. Temporary files are not deleted. Other
functions might implement some logging etc.