Enhancements needed to to be fully compatible with jQuery 3

jQuery 3 is removing a lot of features that have been depreciated for a long time. Some of these are still in use by jQuery Form. https://jquery.com/upgrade-guide/3.0/

I have very little experience working with jQuery Deferred or Promises, so pull requests and assistance in updating jQuery Form is greatly appreciated.

Known issues

1. #541 success and error Deferred methods https://jquery.com/upgrade-guide/3.0/#breaking-change-special-case-deferred-methods-removed-from-jquery-ajax
2. #353 .selector property https://jquery.com/upgrade-guide/3.0/#breaking-change-deprecated-context-and-selector-properties-removed

Other related issues: #527 feature for Deferred #401 Submit only if a promise is resolved