updated .gitignore and README for server-netty

I wanted to build simplepush on my own and I was unable to do so while following provided READMEs. The issue is that when I checkout sockjs branch from netty repository, netty of version 4.0.7.Final-SNAPSHOT is built but I can not build simplepush server afterwards since it uses version 4.0.5.Final-SNAPSHOT.

I realized, from travis build, that there is checkout of ci tag when building netty so I get the right version of it for simplepush server.

I am missing this kind of information in READMEs since it is unbuildable from scratch and one has to investigate further.