add browserify (not yet working) and refactor tree nodes to not be dependent on their parent (currently breaks browser build)

lib/less/tree/attribute.js

@@ -0,0 +1,29 @@
+module.exports = function (tree) {
+
+var Attribute = function (key, op, value) {
+    this.key = key;
+    this.op = op;
+    this.value = value;
+};
+Attribute.prototype = {
+    type: "Attribute",
+    eval: function (env) {
+        return new(Attribute)(this.key.eval ? this.key.eval(env) : this.key,
+            this.op, (this.value && this.value.eval) ? this.value.eval(env) : this.value);
+    },
+    genCSS: function (env, output) {
+        output.add(this.toCSS(env));
+    },
+    toCSS: function (env) {
+        var value = this.key.toCSS ? this.key.toCSS(env) : this.key;
+
+        if (this.op) {
+            value += this.op;
+            value += (this.value.toCSS ? this.value.toCSS(env) : this.value);
+        }
+
+        return '[' + value + ']';
+    }
+};
+return Attribute;
+};