Added network-id configuration option

[slide23]

Adds a 'network-id' configuration option that allows multiple RedisBungee setups to use the same Redis server. Biggest use case would be for separation of production and dev/test networks.

[minecrafter]

Here's some of the issues I see:

• player:* keys are shared, so this isn't a truly clean separation.
• PubSub is not cleanly separated. What this means is that I can broadcast to redisbungee-allservers or another commonly-subscribed PubSub channel and it will be broadcasted to all the networks.
• Forcing a network ID breaks <=0.3.5 compatibility. If you removed the prefix completely, it would work.
• The Lua code path for serverToPlayers() does not accept the network:proxy syntax and needs to be updated.

[minecrafter]

Closing due to no further action.