compress.go: SizeAccumulator added to writer.
compress.go: SizeAccumulator added to writer.

            * Writer now has a SizeAccumulator to determinate the correct
              size written by the underlying compression layer (e.g. frame
              format header from snappy.)
            * AppendToBlockIndex writer method modified to work with
              SizeAccumulator.

disorganizer/brig,disorganizer/brig