Update readme with new multiplayer section + new credits entry, and fix up gradlew start to not overwrite config.cfg
Update networking libraries
Updated protobuf binaries to 2.6.1
Re-generated protobuf java files with compiler v2.6.1
Upgraded te protoc linux version to 2.6.1.

It is a self compiled version.
Remove unnecessary usage of Serializable to fix a permission issue.

This are the only 2 places where we use Serializable and a headless server
log file contained exactly 2 permission violations about Serializable.
Merge PRs #1423 by @msteiger and #1436 by @flo - library updates