Support asynchronous modifications

[mar10]

See #80:

I guess if you initialize from an ajax call, you want to open another request, because you probably have to defer the opening until the request returns.
Something like this: a modification of the beforeOpen event:
we could allow to return a deferred/promise, and the context menu open would be delayed until the promise is resolved.

[mar10]

Some usage example here:
https://github.com/mar10/jquery-ui-contextmenu#howto-modify-the-menu-using-an-asynchronous-request