Feature/history in a line

I reduced dual undo stack into one with stackOffset.
This patch also fixes wrong bookmark implementation.

And changed recording undo stack timing from 'before modifying text' to 'after modifying text'. The first undo stack is recorded while initiating summernote.
tagName보다는 `$.nodeName` 혹은 nodeName을 uppercase로 변경해서 비교하는게 좋을거 같아요.
http://ejohn.org/blog/nodename-case-sensitivity/