Todo list

[HuKeping]

Hi there,
Below is what I supposed to do but things get a little busy these days, however I will keep working on it and if any of you have time to help, PR is welcome :)

• comment for function deleteFixup
• more iterators
• docs for usage
• a function that get the height of the tree
• enhance test
• more examples
• a function that get the successor of the given Item
• add a new file to hold states of the tree. For example, the length, the min, the max, etc. Refactor: add a new file to keep functions for common user #16