Added an easy way to disable the generation of JS Hub proxies

As indicated in #978, the autogenerated JS Hub proxies leak information.

To disable this, set DisableJavaScriptProxies property of the application's
IConfigurationManager to true.
Made the EmptyJavaScriptProxyGenerator class public.
Moved option to disable the generation of JS hub proxies.

DisableJavaScriptProxies is now no longer in the IConfiguration manager.
It has been moved to HubConfiguration which is passed as a settings object
to MapHubs.

HubConfiguration is derived from ConnectionConfiguration which can be passed
to MapConnection.