improvement: Supply thumbnail urls in item request instead of having a separate request

[carolanitz]

currently you need to have additionally to an ODChildrenCollectionRequest ([[[[_oneDriveClient drive] items:itemID] children] request])
another ODThumbnailRequest to get the associated thumbnails ([[[[[_oneDriveClient drive] items:item.id] thumbnails:@"0"] small] request])

this is very inconvenient since the data is not supplied on the items that were requested with the childrencollectionrequest and needs to be manually managed.