Allow specification of different placeholders for different kinds of masks.

[jlesueur]

Allows this:

$.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.

[digitalBush]

At some point I'd like to do groups like mm where masks can define validation and control the caret and value. For instance, user types 9, I can set value to 09 and move caret to position after the 9.

I'm not sure this gets me to that goal.