Add classes for Int and String nodes in the Tree.
Add function TestTreeCreation.
Change name of "ID" property in the doc to "Id". According to
standart rules in C#.
Fix typo in static void Main.
Change TestTreeCreation to private because of rules of well formed
C# doc.
Create PrepareAttr function that return XmlAttributeOverrides object
that will be used to override attributes of serializable objects.
Change position of static main function to the top of the document.
Include xml and xml.serialization libraries. And create functions
for saving and loading tree to the file.
Make tree class and his nodes-classes public.
Delete redundant parameters in XmlElementAttribute constructor.
Test Save and Load functions.