Enhance transparentproxy handling

WCF's RealProxy is `ServiceChannelProxy` which has the same hash code with its TransparentProxy, so the `is` operation can be used as usual. The classic remoting's RealProxy is `RemotingProxy` which doesn't have the same hash code with its TransparentProxy, so the `is` operation performs differently.

The 'GetRealProxy' is a security critical method which is not allowed in partial trusted level. However according to http://support.microsoft.com/kb/2698981, Medium Trust is obsolete.
more info: http://www.asp.net/aspnet/overview/web-development-best-practices/what-not-to-do-in-aspnet,-and-what-to-do-instead#medium

This PR fixed https://github.com/ninject/ninject.extensions.wcf/issues/25