Added missing getters in CPExpression.
Added a type: argument to FunctionExpression initializer used by subclasses to init super with their own type.
Fixed evaluation with bindings dictionary in CPExpression_variable. Allow values in bindings to be a constant or an expression (per cocoa).
Tests: added tests for subqueryExpressions, for variableExpression evaluation where the value in the bindings dictionary can be either a constant or another expression.
Parsing a simple variable expression works (e.g. '$x = 12'), parsing a variable in a combined path (e.g.
'$x.path = 12' won't yet.
