Update README.md
remove several bugs
update setup.py
Change Node ID to support string and interger types; add tree constructor with another tree (deepcopy); add basic methods to measure the tree: size,depth,leaves,contains,is_parent; add deepcopy function to paste method; add parameter checking for several routines in Tree;
disable warnning msgs
add remove_subtree method; add deep option for default tree constructor.
Adding is_leaf method to node.py
Working leaves() method
Added link_past_node method on tree.py to delete a node by linking past it
Fixed two bugs in link past node. Cannot link past the root, and fully expunges a linked past node
Update readme; optimize leaves method