Implement Sass::Util::isAscii() since isascii() is not present on some systems.

isascii() is deprecated according to the manpages http://linux.die.net/man/3/isascii.
See also: http://msdn.microsoft.com/en-us/library/ms235417.aspx

Currently libsass fails to compile on a windows machine running cygwin. Using __isascii() fixes this.

This compiles on linux with gcc 4.8.3, clang 3.5.0-r1 and glibc 2.19-r1.