Emit all files from directories to addedfiles event

[qstiegler]

When you drop a directories into the dropzone, you currently only get the files by the addedfile event. The addedfiles event instead provides at the moment a corrupt file which represents the root directory which you dropped in.

In our project we're disabling dropzone's direct upload and handle the upload and the file handling internally. Therefore we have to know how many files got dropped in total and we can't find this out by the addedfile event cause of we never know what's the last file of the queue.

As the directory reader only works with callback methods, I wrapped the necessary parts in Promises to wait for the complete queue until I emit the addedfiles event. I know that you have browser support for older browsers which do not support Promises but I think in these specific parts of the code it should be okay cause old browsers do not enter these methods as there is the if condition in line 634 which asks for webkitGetAsEntry. I hope you like my solution.

[enyo]

Thanks for this PR. This is using the Promise API and I need to make sure that this polyfills correctly with webpack and babel