Check for net connection

[vanosg]

Prior to running, install.sh should check for a working network connection, otherwise downloads will fail.

While not likely, a user might image an sd card, copy the installer file to the card (not knowing it needs to download more data) and boot the Pi from it without connecting it to a network (or configuring wifi properly, etc) the installer will fail and leave the user in a 'bad' state as some, but not all, of the install did take place.