Updated code and documentation

Added:
1) Walking Methods ( prefix, postfix, symmetrical (in-depth) )
2) Length methods (for Tree node and for the whole Tree branch)
3) Access methods ( [] operators )
4) addChild() expansion for values. ( @example: tree.addChild(0); )
4) Code for testing
5) Documentation in Doxygen style
Fixed:
1) Class lifecycle and memory management (now class creates new TreeNode
in the Stack)