Moves path normalization before trailing slash test
Adds canonical_path helper function

Note that this does *not* collapse x/../y sections into y.
This is by design. If /foo on your system is a symlink to
/bar/baz, then /foo/../quux is actually /bar/quux, not
/quux as a naive ../-removal would give you.
Implements relative sourceMappingURL handling

Adds canonical file resolving where appropriate.

It might should be noted that output_path does not mean
that libsass will write the result to that location. It
is merely used to create file paths relative to the output
file (which defaults to input_path with a 'css' extension).
Improves resolve_relative_path to handle parent references

Accounts for parent references (`..`) in `stripped_base`.