Fix for building on Windows

[bill-chadwick]

The host-examples make target of ol3 currently fails on windows under Cygwin.

To fix this for windows, change https://github.com/openlayers/closure-util/blob/master/lib/util.js to always return POSIX style paths as below.

/**
 * Get the file system path to the Closure Library.
 * @return {string} Path.
 */
exports.getLibraryPath = function() {
  return getDependency('library', config.get('library_url')).replace(/\\/g,"/");
};


/**
 * Get the file system path to the Closure Compiler.
 * @return {string} Path.
 */
exports.getCompilerPath = function() {
  return getDependency('compiler', config.get('compiler_url')).replace(/\\/g,"/");
};