Added option for modal-like popups

This adds three new options to enable modal-like behavior:

* showCloseBtn: Disables displaying the close button.
* enableEscapeKey: Disables dismissing popups by using the keyboard.
* modal: Enables modal behavior for a popup, so it's not dismissable by
    usual means. This is a synonym for setting closeOnContentClick,
    closeOnBgClick, showCloseBtn, and enableEscapeKey to false.
Merge pull request #94 from julen/feature/modal

Added option for modal-like popups