added CreationTimestamp to HttpHealthCheck and TargetPool + refactor War...

...ning

Moves common code for Warning and KeyValuePair our to their own files.
Adds creationTimestamp to HttpHealthCheck and TargetPool
Adds missing live tests for InstanceApi
Improves ParseHttpHealthCheckTest
Nice catch, the properties were package-private from when they were declared in the Metadata class which has implemented get functions. They are now used in the Warning class as well so they should be public.