Add AMD support to jquery.maskedinput

W.r.t. https://github.com/digitalBush/jquery.maskedinput/issues/64

Add AMD API support ( https://github.com/amdjs/amdjs-api/wiki/AMD ) using the boiler plate example here (https://github.com/umdjs/umd/blob/master/jqueryPlugin.js ).

If there is no AMD compliant loader present, it passes the global jQuery object into the module as it did before.