Remove RavenTiming to work with WCF serialization

StackExchange.Profiling.Wcf won't work with StackExchange.Profiling.RavenDb due to serialization problems arising from RavenTiming inheriting from CustomTiming. Without using the KnownType attribute on CustomTiming WCF serialization of RavenTiming will fail. Since the inheritance isn't strictly necessary, I've removed it for the sake of WCF compatibility. The instance methods in RavenTiming can be static methods in RavenMiniProfiler.