SourceMap: Fix relative paths for Windows (#242)

Fixes #242. (ef00e62)

It required converting all paths to lower-case,
the rest of the implementation was there.
This lower-casing is not *locale-aware*, as NTFS is
all case-sensitive when it goes beyond ASCII.

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Fixes output file's default value (bbc02f3)

When the caller provides the input file path but
misses the output path, the default output should
be *input-path-without-extension* plus `.css`.
Case in hand is node-sass, which is currently
missing output file binding (values captured from
`--option`/`-o` switches) in their [binding.cpp](https://github.com/sass/node-sass/blob/3ff1d6f9705e682f271573e394244c943dbfe29b/binding.cpp#L47-L65).
Will fix that issue subsequently.
/cc @andrew, @nschonni.