Fix issue #757 and support placeholder attribute in unsupported browsers

Adding support for the placeholder attribute on text input elements for
browsers that do not support the attribute. In these cases treat the
value of the text field as empty if the value equals the placeholder

Update the handle() function that invokes the submitHandler so that it
returns the result of the submitHandler. This allows the submit event to
continue to propagate (which is required for jquery.unobtrusive-ajax to
function correctly)