Added command line argument for the buffer sie with default value to 8kb
Added command line argument for the buffer sie with default value to 8kb

This leaves the current program behaviour as close as possible to the original behaviour
(with the exception of the byte by byte feeding to the hash algorithm which was probably
a bug)

duplicati/duplicati,mnaiman/duplicati,duplicati/duplicati,mnaiman/duplicati,mnaiman/duplicati,duplicati/duplicati,mnaiman/duplicati,mnaiman/duplicati,duplicati/duplicati,duplicati/duplicati