Grammar update: lock expression

Hey folks, not sure if there's any interest, but figured I'd pass this along. Adds the ability to limit an expression by appending a filter expression to the end of a metric or event expression. 

This allows us to create an interface to the Cube evaluator and still allow the client to provide the initial expression. Our application just needs to append the lock expression before forwarding to our (well secured) evaluator.

Feedback's awesome, this is one of my very few forays into node.