Added ability to customize mask button

Can be used to replace/remove default text, place img icons, add some classes, etc.
For example, to use icons from [Font Awesome](http://fortawesome.github.io/Font-Awesome/icons/) instead of abc text:

        $("#inputId").passField({
            maskBtn : {
                textMasked      : "",
                textUnmasked    : "",
                className       : "fa",
                classMasked     : "fa-eye",
                classUnmasked   : "fa-eye-slash"
            }
        });