Issue 25 resolve relative paths

This fixes issue #25 

I'm using the grunt api `grunt.file.isAbsolutePath` which is just a simple regex on the file path.

I also [found](http://stackoverflow.com/a/17521358/53715) the function below which also works, but went with the grunt api instead.

    function isRelative(p) {
        var normal = path.normalize(p);
        var absolute = path.resolve(p);
        return normal != absolute;
    }