replace_extension now returns it's input for directories, with a warning.
get_extension now returns a named character vector, with the original input for the names.
standardize_path now behaves more consistently across OSes.  In particular, the paths should be absolute on Unixes as well as Windows.
strip_extension now returns directories without a trailing slash, and has the original inputs for names.
decompose_path now restandardizes directory names to avoid weird edge cases with symbolic links in Unix.
Tests now work on Unixes.
Replaced [ indexing with $ indexing.