Refactor tests. Uses unittest module features. Uses Mock library.
Merge pull request #17 from johntconklin/feature/normcase

Use os.path.normcase() to normalize case.
Determine untracked directories, and remove those as a whole rather
than tracking the individual untracked files they contain.
Rework the way we remove subdirectories from traversal
Compute depot folders at the same time as computing depot files.
Handle excluded files and directories within get_untracked_files().

This change allows us to avoid traversing into excluded directories.