[ADF-643] upload enhancements (#1949)

* rework folder uploading

- flatterns hierarchy on folder upload
- performs a single traversal for the entire folder heirarchy and ends with a comple file list
- allows now dropping folders on existing folders
- overall code improvements

* fix unit tests

* readme updates

* clean old and unused code

* code cleanup

* limit concurrent uploads

* update code as per review

* fix upload button for Safari

* fixes for Safari

- Safari compatibility
- code updates based on review

* fix code

* fix unit tests

ng2-components/ng2-alfresco-upload/src/components/upload-button.component.ts

@@ -211,6 +211,7 @@ export class UploadButtonComponent implements OnInit, OnChanges {
         return Observable.throw(error || 'Server error');
     }
 
+    // TODO: move to AlfrescoContentService
     getFolderNode(nodeId: string): Observable<MinimalNodeEntryEntity> {
         let opts: any = {
             includeSource: true,