Improve JS primitive types handling and bug fixes

This release introduces separate representation for undefined value and fixes some bugs. Please, see change list below for more details.

* - BC-breaking or potentially BC-breaking changes

Changes to public API and other important changes which may affect end-user:

• * Make V8\Isolate::ThrowException() method void;
• * Remove non-documented V8Isolate::ContextDisposedNotification();
• * Make V8\Value, V8\PrimitiveValue and V8\NameValue abstract;
• V8\Isolate time limit affects js runtime only;
• Introduce separate class for undefined value - V8UndefenedValue;
• Fix V8\Tempalte::Set() to do not accept non-primitive values;
• Remove all private properties from V8\Isolate;