Updated README.md

Updated for cleaner and simpler instructions. The library, example_framework, example_executor and all dependencies are now fetched and built with one command.  
Also, be advised that **protoc-gen-go** is in fact not needed by mesos-go users, since the generated protobuf sources are included within this repository.  
I removed the mention of G++ because I have never in my life seen GCC packaged without G++.