[Proposal] Make buffer size configurable

[randallpittman]

Especially for transferring to a network share, it may be advantageous to have a larger buffer size. I got a 33% speed increase transferring a 740 G file over gigabit to a samba share (Win XP VM to CentOS host) by bumping up the buffer size to as much a 4MiB. I propose making a module-level variable in pyrocopy.py that is used in _copyFile() to set the buffer size in kiB.

[randallpittman]

Default can still be 16, ofc. This seems to be a good value for same-disk operations.

[caskater4]

PR merged. Thanks!