Ert 712 ert socket

This PR creates a very simple server instance which can communicate with elco.

1. ErtServer : this holds on to a ert main instance, it is controlled with json objects.
2. ErtSocket: this hold on to an ErtServer instance sends json strings back and forth over tcp.
3. ert_tcp_server - small 'program' based on ErtSocket which elco can communicate with.

The current PR is quite limited, but functional.