added sublime syntax for gcov files

Allows quick inspection of .gcov files, since untested lines are highlighted
using the theme's "invalid" marker.
implemented relative and normalize path

Added two functions to filesystem/Path.hpp. relativePath() makes one path
relative to another. normalizePath() resolves ../ in a path, removes excess ./
and //, and removes trailing slashes. Both are handy for writing the user tools
where we would like to be friendly with our paths.