Matcher to check whether json contains certain field without value check

See #6, release log:

• 34f5de5 by @g4s8: #6 - extended JsonValueIs
• d7c965c by @g4s8: #2 - JSON array matchers
• 12e70ba by @g4s8: Update README.md
• 8be8521 by @g4s8: Update README.md

Released by Rultor 1.68.1, see build log