Fixes some tree rendering problems where lines were not properly drawn
at some levels during some circumstances.
* Change the API back to only allow one root.
* Now uses a stack based approach to rendering instead of recursion.
* Removes the need for measuring the whole tree in advance.
Leave this up to each child to render.
Add extension method for specifying horizontal and vertical padding.
Similar constructor for `Padding` already existed.
Update documentation for table column appearance (padding).
Update
`Should_Render_Padded_Object_Correctly_When_Nested_Within_Other_Object`
test to use new extension method.
