Replace ZeroClipboard with clipboard.js.

Things left:
1. clear the selection after copy
2. add messages for success and error

docs/assets/scss/docs.scss

@@ -57,7 +57,7 @@ $bd-info:           #5bc0de;
 @import "team";
 @import "browser-bugs";
 @import "brand";
-@import "zeroclipboard";
+@import "clipboard-js";
 
 // Load docs dependencies
 @import "syntax";