New: added support for per-file abort.

If the table cell view's class is set to JQueryFileUploadTableCellView, you can connect an abort button to its -abort: method.

The abort: method will abort that file's upload and call a new delegate method, -fileUpload:uploadWasAbortedForFile:.

Also cleaned up unused stuff from the cell-based demo, and moved the byte count transformer to a new class in a separate file, JQueryFileUploadByteCountTransformer.j.