proof of work - which data is used

See #27, release log:

• d8cb99c by @yegor256: #27 fixed typos with score cal...
• 31500bd by @yegor256: #29 typo
• ed25c4b by @yegor256: #29 typo
• e625673 by @yegor256: #29 typo
• 70c173e by @yegor256: #29 travis cfg

Released by Rultor 1.67.4, see build log