Add estimated remaining time

Really simple patch to add an ETA when this one is greater than 0 (just to not print an ETA of 0 when we don't know the destination size like on a wget download)

Preview of ETA on a scp copy
> $ ./cv -c scp -w
> [26040] scp [...]/ubuntu-14.04-desktop-i386.iso 4.5% (44 MiB / 970 MiB) 3.0 MiB/s eta 0:05:08