Fixed #445 - ChannelFactory does not support the contract ISubProcessProxy

In CefSharp.Internals.SubProcessServiceHost.KillExistingServiceIfNeeded in empty catch there was Exception "ChannelFactory does not support the contract ISubProcessProxy as it defines a callback contract with one or more operations.  Please consider using DuplexChannelFactory instead of ChannelFactory."

Commit https://github.com/ivan-sam/CefSharp/commit/e3038a00921245b0fbd67f1f25b0c800b05bac10 of this pull request fixes https://github.com/cefsharp/CefSharp/issues/445

Commit https://github.com/ivan-sam/CefSharp/commit/8d7f9365be29c0d777fb4bcfbefda1a889b3e6a9 and following add full page image snapshot feature. Please consider these commits as well. Finally, I moved ChromiumWebBrowserWithSnapshotSupport class to CefSharp.Wpf.Example project as it is essentially an example of how one can use base control for his/her needs.