SourceMap: Resolve path w.r.t output directory

With this, the last failing test passes on Windows.

Thanks @mgreter for insights. We should close the libsass PR: https://github.com/sass/libsass/pull/477.

The only test failing on both 0.10 and 0.11 is stdin, as discussed here: https://github.com/sass/node-sass/pull/410#issuecomment-54213682