Adding field

lib/carto/tree/field.js

@@ -0,0 +1,13 @@
+(function(tree) {
+
+tree.Field = function Keyword(value) {
+    this.value = value;
+    this.is = 'field';
+};
+
+tree.Field.prototype = {
+    eval: function() { return this },
+    toString: function() { return '[' + this.value + ']' }
+};
+
+})(require('../tree'));