added base 64 encoding for rhino, fixes urls.less test case

lib/less/encoder.js

@@ -0,0 +1,4 @@
+// base64 encoder implementation for node
+exports.encodeBase64 = function(str) {
+    return new Buffer(str).toString('base64');
+};

lib/less/functions.js

@@ -579,9 +579,8 @@ tree.functions = {
                     '<rect ' + rectangleDimension + ' fill="url(#gradient)" /></svg>';
 
         if (useBase64) {
-            // only works in node, needs interface to what is supported in environment
             try {
-                returner = new Buffer(returner).toString('base64');
+                returner = require('./encoder').encodeBase64(returner); // TODO browser implementation
             } catch(e) {
                 useBase64 = false;
             }