jquery plugin initializer doesn't return promise object

[embeepea]

The Multigraph jQuery plugin is documented on http://multigraph.github.com/docs/js.html to say that it returns a promise object:

var promise = $(selector).multigraph({...});
promise.done(function() { ... });

However, it does not really seem to be returning a promise, because the above doesn't work. It is probably returning the jQuery object instead, since that's the standard behavior fo jQuery plugins. In order to use the promise, we have to do something like the following code from fcav:

https://github.com/nemac/ol-fcav/blob/0063efd176a0aacbd6ccf9174afa54b8df5620ca/fcav.js#L1230-L1239

Resolve this disparity by either:

* changing the plugin so that it really returns a promise, or
* deciding that the standard plugin behavior of returning the jQuery object is better, in
  which case the documentation mentioned above should be changed.