SourceMap: Fix relative paths for Windows (#242)

[am11]

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.
Will fix that issue subsequently.
/cc @andrew, @nschonni.

[am11]

Thanks for merging this @akhleung. 👍

[mgreter]

I have a regression with this merge on windows.

libsass\file.cpp: In function 'std::string Sass::File::resolve_relative_path(const string&, const string&, const string&, bool)':
libsass\file.cpp:91:71: error: no matching function for call to 'transform(std::basic_string<char>::const_iterator, std::basic_string<char>::const_iterator, std::basic_string<char>::iterator, <unresolved overloaded function type>)'

See the <unresolved overloaded function type>.
Problem is explained here: http://stackoverflow.com/questions/1679245

The fix is pretty easy. Just change tolower to ::tolower.

[am11]

Oh I see. Actually I first put it then removed the scope resolver. I was just trying to make code look smarter which VC2013's compiler didn't mind. :)

Will fix it. Thanks!