Fixes unicode filepath handling on windows

In reference to: https://github.com/madskristensen/WebEssentials2013/issues/1406

Libsass currently cannot handle unicode file paths. Although this is not really the fault of libsass. There is no way to access unicode paths that have codepoints not available in the current codepage, as the ANSI functions will only work with the current codepage [[*]](http://msdn.microsoft.com/en-us/library/windows/desktop/dd374089%28v=vs.85%29.aspx).

One way to solve this would be to replace all ANSI IO functions with the windows specific ones (like CreateFileW etc). Since libsass only reads data from the filesystem, we can use `shortnames` to access files with ANSI functions. This Patch does exactly that.

This patch will need the windows headers if `_WIN32` is defined. I guess it should compile safe with different compilers on windows. Although I just tested it with gcc version 4.6.3 (gcc-4.6.3 release with patches [build 20121012 by perlmingw.sf.net]).