Ensure that the Program node may still have leading comments.

https://code.google.com/p/esprima/issues/detail?id=536
Update ChangeLog for 1.2.1.

https://code.google.com/p/esprima/issues/detail?id=514
Stable version 1.2.1.

https://code.google.com/p/esprima/issues/detail?id=514