Refactor 80-character ruler for speed

Don't create a div for each line. Instead, calculate the width of the
gutter (line numbers) and take the padding of the "code" cell into
account. This approach results in vastly improved performance when
reviewing large patches, positions the ruler at the same pixel
position as before, and barely changes the visual appearance.
Refactor to use thead instead of pseudo-element
Merge pull request #95 from dreditor/fast-ruler

Refactor 80-character ruler for speed