[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-drag-area.component.spec.ts

@@ -128,7 +128,7 @@ describe('UploadDragAreaComponent', () => {
         component.showUndoNotificationBar = jasmine.createSpy('_showUndoNotificationBar');
 
         fixture.detectChanges();
-        let fileFake = new FileModel(<File> {name: 'fake-name-1', size: 10, webkitRelativePath: 'fake-folder1/fake-name-1.json'});
+        let fileFake = <File> {name: 'fake-name-1', size: 10, webkitRelativePath: 'fake-folder1/fake-name-1.json'};
         let filesList = [fileFake];
 
         component.onFilesDropped(filesList);