Use Common Parse Tree Nodes

[Andrei15193]

Currently, the parse result is specific to the Creole parser right now which calls specific methods on the visitor. This can be further improved by having a common Parse Tree representation that accepts the visitor. The Parse Tree Nodes act as method selectors for the visitor where the visitor is responsible for traversing the tree by calling Accept(this); on child nodes which will in turn select the specific method from the visitor.

This will enable the parse tree to be reused between parsers (Creole, Markdown etc.) combined with the visitors which can generate different results (HTML, XML, JSON etc.).