used more efficient method for .empty()

src/publicapi.js

diff --git a/src/publicapi.js b/src/publicapi.js
index 615b0aa..63cd46f 100644
--- a/src/publicapi.js
+++ b/src/publicapi.js
@@ -161,7 +161,12 @@
       return this;
     };
     _.empty = function() {
-      this.select().__controller.backspace();
+      var root = this.__controller.root, cursor = this.__controller.cursor;
+      root.eachChild('postOrder', 'dispose');
+      root.ends[L] = root.ends[R] = 0;
+      root.jQ.empty();
+      delete cursor.selection;
+      cursor.insAtRightEnd(root);
       return this;
     };
     _.cmd = function(cmd) {