Error when calling the function "destroy" which did not remove the ev…

[rafgal]

Error when calling the function "destroy" which did not remove the events assigned to the sandbox is repaired.

• Add a variable to store the function associated with the events.
• Change the constructor to assign the function to the variable.
• "BindEvents" and "unbindEvents" was changed to make use of the variable created.

[jtsternberg]

Just as a heads up, I've forked this repo, and merged this PR in: https://github.com/zao-web/texthighlighter