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 
You're using the name `length` in the docs, can you change all `distance` references to `length`?

Also, the error message might be clearer if it were something like:

```Could not convert length to a numeric type: %s```