Moved setting of the time right after the network has come online.

Also removed the old commented out code, since that's what a VCS is for.
Add logging functionality.

Everything that gets logged on the screen is now also logged to file.
With successful installations, the log file will be at
/var/log/raspbian-ua-netinst.log.
When installation fails, the log file will be put on the sdcard (normally
mounted on /boot) and will be named raspian-ua-netinst-<datetimestamp>.log.