Abstract envelopes

See #24, release log:

• 00a8899 by @maxonfjvipon: comments
• cbbef36 by @maxonfjvipon: remake interfaces
• 3656526 by @maxonfjvipon: make all final where it's poss...
• 7d1ce34 by @maxonfjvipon: one return where it's possible
• 4708329 by @maxonfjvipon: abstract envelopes + ArrWith +...
• 90b889e by @maxonfjvipon: Update README.md

Released by Rultor 2.0-SNAPSHOT, see build log