Allow file copy buffer size to be configured

Files are copied with a fixed buffer of 4KB (because `ByteStreams.copy` is used) and that causes some reads and writes, e.g. those to Windows shares, to be very slow.

Allow this the buffer size to be configured to allow the user the tweak the performance.

_N.B.:_ `ByteStreams.copy` is deprecated and will be removed in Guava 18.0.
