move functions into its own folder and give it a set interface

lib/less/functions/function-registry.js

@@ -0,0 +1,18 @@
+module.exports = {
+    _data: {},
+    add: function(name, func) {
+        if (this._data.hasOwnProperty(name)) {
+            //TODO warn
+        }
+        this._data[name] = func;
+    },
+    addMultiple: function(functions) {
+        Object.keys(functions).forEach(
+            function(name) {
+                this.add(name, functions[name]);
+            }.bind(this));
+    },
+    get: function(name) {
+        return this._data[name];
+    }
+};