Add JavaScriptTimerProvider.

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

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

This patch seperates the timer implementation from engine. After this
patch, who uses BeyondJavaScriptEngine has to implement timer and pass
the timer to engine.