initial commit to support GCE LB

add support targetPools, forwardingRules and httpHealthChecks API
add expectedTests and LiveTests for the above API
Do we really need three methods to get a concrete page? Could you refactor all three into a single one that accepts the ListOptions object? The "pageToken" parameter could be moved to that options object.
Also rename the method to the same name than the one that lists all records. In general prefer method overloading for this kind of list methods.