Add JavaScriptTimerProvider.

JavaScriptTimerProvider will be used in the next patch to seperate timer
implementation from engine.
Implement timer for GamePlugin.

Implement JavaScriptTimerProvider with Akka actor.
Implement timer for JavaScriptShellConsole.

Implement JavaScriptTimerProvider with java.util.Timer.
Seperate JavaScript timer implementation from engine.

Currently, JavaScript timer strictly depends on Akka Actor system although
other parts of JavaScript engine doesn't depend on Akka.

This patch seperates the timer implementation from engine. After this patch,
BeyondJavaScriptEngine takes a JavaScriptTimerProvider instance.