Added mask, custom body view and custom alert-dismiss buttons.

1. New: addsMask allows you to set/reset the mask displayed behind the alert. Default is YES, which means the alert displays a semi-transparent mask over the background.
2. New: customBodyView property allows to set a custom view as the alert body (as opposed to setting only the text body). This allows finer-grained control over e.g. the background image of the body area, sepatately from the alert background image.
3. New: option to set buttons which _do not_ dismiss the alert automatically when tapped.