use the main worker in ringo shell instead of a separate new worker

reason: when loading a module in interactive mode (i.e. `ringo -i mymodule.js`) the module would be evaluated
using the main worker, but any calls of the module methods would be executed in the shell worker, which is
a different one. this makes using eg. `setTimeout` in module methods impossible because they fail with
`Current thread worker differs from scope worker` exception

src/org/ringojs/tools/RingoShell.java

@@ -31,7 +31,6 @@
 import org.ringojs.engine.RingoConfig;
 import org.ringojs.engine.RingoWorker;
 import org.ringojs.engine.ScriptError;
-import org.ringojs.repository.Repository;
 import org.mozilla.javascript.*;
 import org.mozilla.javascript.tools.ToolErrorReporter;
 import org.ringojs.repository.Resource;
@@ -45,8 +44,6 @@
 import java.util.List;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
-import java.security.CodeSource;
-import java.security.CodeSigner;
 
 /**
  * RingoShell is a simple interactive shell that provides the
@@ -60,7 +57,6 @@ public class RingoShell {
     Scriptable scope;
     boolean silent;
     Path history;
-    CodeSource codeSource = null;
     final String PROMPT = ">> ";
     final String SECONDARY_PROMPT = ".. ";
 
@@ -73,7 +69,7 @@ public RingoShell(RhinoEngine engine, Path history, boolean silent)
         this.config = engine.getConfig();
         this.engine = engine;
         this.history = history;
-        this.worker = engine.getWorker();
+        this.worker = engine.getMainWorker();
         this.scope = engine.getShellScope(worker);
         this.silent = silent;
     }