Seperate JavaScript timer implementation from engine.

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

This patch seperates Timer implementation from engine. After this patch,
who use BeyondJavaScriptEngine have to implement timer and pass the
timer to engine.
This patch contains timer implementation for GamePlugin, but doesn't
contain timer for JavaScriptShellConsole.