Added methods uploadDone and allUploadsDone
uploadDone and allUploadsDone may return deferreds
Added failure test cases
Added subclassing test
Pulled runTransferCommand up to _TransferBuildStep

Formerly runUploadCommand of MultipleFileUpload.
The runTransferCommand function adds a callback to extract the result
from the command instance and (optionally) ann error callback to
cancel the command's writer.
Added test cases for upload errors