Fix relative paths (fixes #5)

The new test case (/test/fixtures/relative) creates a relative URL example for all supported content types: HTML, JS, CSS.
We need the following instead to be Windows compatible:

    // Returns hashed url relative to file (this.name)
    Dependency.prototype.resolveAsHashedUrl = function (url) {
        var resolvedUrl = this.resolve(url);
        var hashedUrl = this.tree.get(resolvedUrl).name;
    
        return path.normalize(
            path.relative(
                path.dirname(this.name),
                hashedUrl
            )
        ).split(path.sep).join('/');  // Because these are URLs
    };