Add newick format

Ready for review

I still need to add `read` and `write` as well as deprecate `to_newick` and `from_newick`

@wasade I am particularly interested in any comments you might have.

fixes #627 
Can you add a quick summary of the different pieces of info that can be attached to a node (e.g., labels and distances)? It'll make the next two paragraphs clearer.