Introduce ternary logic in Cypher

o Make Predicate return Option[Boolean] instead of Boolean
o Introduce isTrue that returns false for all cases except when the
   predicate returns Some(true)
o Add logic for IN, ANY, ALL, SINGLE and NONE to handle nulls
o Add a new section on working with nulls to the manual
Merge pull request #1458 from systay/ternary-logic

Introduce ternary logic in Cypher