Also added methods and documentation for the new methods:
-trueExpression, -falseExpression and -expressionBlock.
Completed documentation for -predicate and -arguments.
When the predicate part of a subquery expression was containing
variables, the substitution was ignored.
Now, the subpredicate can contain variables that will be substituted
when calling evaluateWithObject:substitutionVariables:.
Added expression test and predicate parsing test.