Adapt to hash compatibly with draft standard

Namely [this one] [s11n] which you've raised no objections to and hopefully approve of! :)

 [s11n]: https://github.com/olalonde/proof-of-liabilities#serialized-data-formats-work-in-progress--draft

This is with a view to adding support for JSON de/serialisation in a further pull request.

Also:
 * accept uid/nonce as alternative to externally produced hash, to encourage
   standard hashing format
 * be sure to provide original data to validation, rather than feeding
   data obtained from tree right back in
 * use `sum` not `value`, and `hash` not `hashdigest`, (where that doesn't
   conflict with builtins) to match terminology in standard and in other implementations
 * update README to match, including suggesting following draft standard
 * change test to construct `Node` with uid/nonce leaves, since internal node
   hashing already tests externally-provided-hash constructor
 * add `__str__` for slightly pretty-printing `Node`
 * be more verbose in test output
 * update link to point to preferred, more verbose/pretty description