Handle multiple players + Plugin system + Webdis plugin

This switches from a single-player map to a multiple-players map.
Updates are moved to a file imported from the "plugins" directory to make it easier to plug in new means of sharing data.
The default is the "local" plugin which has the same behaviour has before.
I've also added a "webdis" plugin to share positional data through a central redis server (with webdis for the HTTP layer).