Make Cloudfiles copy memory efficient too (#419)

Make Cloudfiles copy memory efficient too (#419)

It seems that (our implementation of) cloudfiles.write() takes
all existing data and appends write(data) to it, sending the
full monty over the wire everytime. This would of course
absolutely kill chunked writes with some O(1^n) performance
and bandwidth usage. So, override this method and use the
Cloudfile's "send" interface instead.

Also make the Cloudfile file wrapper an iterator that allows us to
simply do "for data in cloudfile:" which will stream the data in a
memory-efficient way.

DO NOTE THAT THIS PATCH IS COMPLETELY UNTESTED DUE TO LACK OF SETUP
PLEASE REVIEW AND VERIFY.