Checking for native browser support in jQuery.fn.placeHolder

Need to check for native browser support in jQuery.fn.placeHolder method; existing implementation checks only when the plugin is defined.

This patch handles scenarios in which a DOM structure is built dynamically (say, with a Backbone view) and then a post-render callback executes jQuery.fn.placeHolder passing in a jQuery set containing references to these new DOM elements. At that time, the config object's forceApply value should be checked along with native browser support.