Image upload bug

Hi

I found that my uploaded images were never appearing after they had been successfully uploaded to the server.

It turns out that `this.$el` at https://github.com/orthes/medium-editor-insert-plugin/blob/master/src/js/medium-editor-insert-images.js#L109 was always `undefined` (since `this` means something else in the ajax complete callback) and so `var $progress = $('.progress:first', this.$el)` turned out to be selecting another progress bar I had on my page which had nothing to do with this plugin.

This pull request passes the right element into the `uploadComplete` function so that the correct progress bar is selected.

Note: I haven't yet grunted the project in this pull request, I can do that if needed.