Added __server_state_counter__ in serverinfo_js.

When designing experiments with between-subject factors, it can be useful or even essential to have access to the counter in server_state.  See [this discussion]](https://groups.google.com/forum/#!topic/ibexexperiments/-6FPcQIE5S0) on the mailing list.  This PR adds the variable `__server_state_counter__` which can be used in items.js.