option to set blocksize

[seb711]

Added an option to set the blocksize used in btrblocks based on the analysis made. With that the default Blocksize is still 1 << 16. But as the analysis showed a better performance could be achieved by reducing the blocksize. To better evaluate this analysis I've modified the decompression-speed script; that on the one hand now measures the correct decompressed size and on the other hand can verify the result with the binary uncompressed data.