resolving differences between older and modern browsers

## Resolves
* https://github.com/fabien-d/alertify.js/issues/83
* https://github.com/fabien-d/alertify.js/issues/81

## Notes
* Removed `setTimeout` function used to give focus to dialog button or textbox. Replaced with a `transitionend` event listener or direct focus (for browsers that don't support CSS transitions.
* Added unique cover hidden class `alertify-cover-hidden`
* Separated function callback for prompt and other dialogs. Some browsers had issues having a callback paramater that didn't exist.