Max-Age and Expires headers

See #25, release log:

• 57e5832 by @yegor256: #25 polish
• 4d4e8ed by @yegor256: rultor assets fixed
• e8f3b44 by @yegor256: #25 doc
• a7e9794 by @yegor256: #25 typo
• 7890b7c by @yegor256: #25 doc
• 902a5cc by @yegor256: #25 cache forever

Released by Rultor 2.0-SNAPSHOT, see build log