Support for parameter passing in comparison subquery

[sileix]

The current implementation does not have any variables in the outer scope available in subquery - which means neither parameter passing nor filters using variables from parent query is not supported. This should probably go after #33

[gcampax]

I don't know that #33 is a blocker on this. #33 will definitely not happen by the alpha milestone, and depending on whether we can do it in a compatible way it might not happen by ThingTalk 2.0 either.