Scoped @import (plugin) functions loading

- Limited @import (plugin) support to add/addMultiple of functions
- Altered @import (plugin) loading to support browser
- Support proper closure scoping of @import (plugin) loaded functions

test/less/plugins/test.js

@@ -0,0 +1,4 @@
+
+functions.add("test", function() {
+    return new tree.Anonymous( "PASS" );
+});