moved the observable feature to the mainproject

- moved observable functionality to SignalR.Client.TypedHubProxy
- removed project SignalR.Client.TypedHubProxy.Observable
- renamed some classes and interfaces (no backward compatibility)
- adjusted unittests
deleted SignalR.Client.TypedHubProxy.Observable project
renamed methods for IHubProxy and HubConnection

- AsHubProxy<T1,T2>
- AsObservableHubProxy<T1, T2>