More work on S3 multipart uploads, since the aws library got fixed today
to return the ETAGs for the parts. I got multipart uploads fully working,
including progress display.

The code takes care to stream each part in from the file and out the
socket, so I'd hoped it would have good memory behavior. However, for
reasons I have not tracked down, something in the aws library is causing
each part to be buffered in memory. This is a problem, since I want to
use 1 gb as the default part size.
