add a REPL interface for debugging

This adds a REPL interface to the worker processes (reachable via TCP
socket) that exposes access to the GSJS admin object, the global model
API and the persistence layer. A simple client is also included (start
via `npm -s run repl`).
make stacktrace length limit configurable
Merge pull request #45 from yelworc/debug-repl

add debugging REPL