Autoupdate mechanism

[calmh (Jakob Borg)]

While I agree with @sil in principle, I’ve added an optional, non-automatic upgrade system.

[v0.8.3] Self upgrade

v0.8.3 includes support for performing basic self-upgrade. To perform an upgrade (i.e. when v0.8.4 or higher is out), run syncthing -upgrade. It does not happen automatically. When called with the -upgrade flag, syncthing will Check Github for the latest release and compare to the current running version. If the release version isn’t higher than what we’re running, exit. Download the .tar.gz for the corrent OS and architecture and save the syncthing binary to a temporary location. If ev…