Cleanup / Added Pylintrc

Added an extra check to avoid a possible division by zero error in the report generator.

Added comments for `measured_lines()` returning `None` to indicate
that all lines are covered by default.

Added tests that would have caught this: https://github.com/edx/diff-cover/issues/23
As far as I can tell, the bug got introduced, then fixed, without anyone noticing :)

Fixed a few small style issues (mostly line length).

Added the pylintrc file from edx-platform.