Added renderFile

The renderFile function allows you to specify a filename to be rendered.

The function automatically caches the parser function and will use it the next time the same file is rendered.

It is incredibly useful when your haml templates are stored in files.

        haml.renderFile('layout.haml', 'utf8', options, function(err, data) {
            if (!err) {
                // stuff
            } else {
                // stuff
            }
        });