relay fixes
Use existing method to create message
Fix restoring content references (omit NPE)
Fixed documentation and some NPE with message handling
Merge remote-tracking branch 'tomp2p/master'

Conflicts:
	nat/src/main/java/net/tomp2p/relay/RelayRPC.java
	nat/src/test/java/net/tomp2p/relay/TestRelay.java
Logging improvements and message optimization
Don's just use any sender / recipient socket