Allow specification of different placeholders for different kinds of masks.

Allows this:
```javascript
$.mask.definitions['d'] = $.mask.definitions['9'];
$.mask.definitions['m'] = $.mask.definitions['9'];
$.mask.definitions['y'] = $.mask.definitions['9'];
$.mask.placeholders['d'] = 'd';
$.mask.placeholders['m'] = 'm';
$.mask.placeholders['y'] = 'y';
$('.date-field').mask('mm/dd/yyyy');
```
Or placeholders can be specified using the options parameter.

also adds a class "placeholder" when the element is empty.

There's some whitespace changes that I can remove if it bothers you, and there may be better things to do than add a placeholder class when empty (such as trigger("mask:empty")). But I think the placeholder implementation is solid. I can also split the changes up if you prefer.