GCE Loadbalancer Continuation

Thanks to @andreaturli for kicking this off in PR #22 

This isn't ready yet but I wanted to seek comments on the TargetPoolApi and associated TargetPoolCreationOptions. 

There is redundancy between the TargetPoolCreationOptions and the TargetPool resource class. I have taken the rout of having a separate class with redundant code. Happy to change, what do people think? 

I plan to do the same thing for the ForwardingRuleApi