Make copying to/from storage systems memory efficient (#419)

The copy_locally and copy_local_to_storage (very inconsistent terms BTW)
were simply slurping in everything in RAM and writing it out at once.
(the copy_locally was actually memory efficient if the remote system was local)

Use shutil.copyfileobj which does chunked reads/writes on file objects.
The default buffer size is 16kb, and as each chunk means a separate HTTP
request for e.g. cloudfiles, we use a chunksize of 4MB here (which has
just been arbitrarily set by me without tests).

This should help with the failure to upload large files issue #419.

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