Adapt to hash compatibly with draft standard

Namely the one at:
   https://github.com/olalonde/proof-of-liabilities#serialized-data-formats-work-in-progress--draft

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 'sum' and 'hash'
 * 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 stale-ish link to description
Merge pull request #4 from zw/standards-1

Adapt to hash compatibly with draft standard