Added emit function  to Dropzone

See the docs for some [examples using emit](https://github.com/enyo/dropzone/wiki/FAQ#how-to-show-files-already-stored-on-server)
I can't find any other usage so this covers the two events that I am aware of: `addedfile` and `thumbnail`
It looks like it depends on the event name.

https://github.com/enyo/dropzone/blob/master/src/dropzone.coffee#L827
https://github.com/enyo/dropzone/blob/master/src/dropzone.coffee#L912
https://github.com/enyo/dropzone/blob/master/src/dropzone.coffee#L576
https://github.com/enyo/dropzone/blob/master/src/dropzone.coffee#L643

What is the proper way to do this with typescript?