add Range support

[benoitc]

Add range support to be able to fetch only partial blobs or stream them.

[benoitc]

The new internal coffer api willl be:

Options = [{window, 1024}, {timeout, infinity}, {range, Range}, {ranges, Ranges}]
Range = {start, end}
Ranges = [Range]

new_stream(Storage, BlobRef, Options)