Fixes regressions introduces in previous merges

aa023009a72a39a26bf705f6853cdadb4853fba9 - omit_source_map_url
61edd1457dcaeb70e1ecfc225b43891e12450fae - FS Case Sensitivity
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`.
Tweaks `make_canonical_path` for better performance
Improves source map output on `compile_string`

Add a source entry for stdin. This mimics exactly the
behavior of the Google Closure Compiler on stdin data.
Initializes SourceMap with path relative to output

Points to working directory (ending with a slash) if no
input file path has been given (i.e. on 'compile_string').
Uses Sass::File namespace
Merge pull request #455 from mgreter/srcmap-regressions

Fixes regressions introduced in previous merges