Support for promises

[skwasjer]

Support for promises would allow better integration with frameworks and coding styles that make use of promises.

Requirements

• Don't break the current API
• 0 (zero) callback handlers should be registered in both:
  • show(options)
  • show(options, successCallback, errorCallback)
    If this is the case, then we can return a promise from show(options)
• promise resolve is same as options.success(date). options.cancel() should be supported also by resolving with undefined (no) argument. This behavioral difference must be well documented.
• promise reject is same as options.error(err)
• the implementation should be seamless and not break current behavior (when no promise support f.ex. is available or when callbacks are supplied).
• the plugin will not include a polyfill for Promise, this is a choice the app developer can make for himself
• ECMA5 compatibility

This is open for anyone to pick up. Improvements to these requirements welcome too.