Deferred and Event handlers

For my application, I needed a couple different hooks on the open and close of the modal. I added two configuration options for open and close event handlers. They are called in the activate and deactivate functions respectively. I also added a return to the activate function. It returns a promise that will resolve when deactivate was called with the parameter that deactivate would be called with. The promise would only fail if the HTML could not resolve and therefore the modal would not ever open. I made changes to the readme as you can see.