Add an ability of subclassing to GChart implementations about charts4j HOT 2 OPEN

Why do you have to do this via subclassing? Why not use object composition. 
Simply write a class that has a factory method that creates a chart with the 
styles you want. I was inspired by Josh Bloch's Effective Java when I wrote 
this library. In short, inheritance breaks encapsulation.

In fact, an inheritance sometimes breaks encapsulation.  So I was tried to 
implement my specialized charts by not inheritance but composition.

At that time, I realized that my charts have hierarchical structure and "is-a" 
relations each other.  The formal solution for this situation is composition.  
I know I should build my own class structure same as tree of GCharts.  However, 
as a design decision, I may select inheritance to prevent a complexity more 
than needed.  I think that it would be a better not to inhibit potential to 
select inheritance.

Humm...  I have also read Effective Java.  So I know the risk of subclassing 
abuse and think you may be right.  I'll try to composy my hierarchical 
structure again.

Thank you.