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`
Added more emit() overloads
Merge pull request #2701 from styfle/patch-7

Added emit function  to Dropzone