Fixed issue introduced with DomCapture

In PR https://github.com/TestStack/TestStack.Seleno/pull/168, a new Castle Dynamic Proxy Interceptor was introduced to output the `DOM` on exception. This unfortunately broke the existing feature that captures a screenshot.This was noted by @robdmoore after https://github.com/TestStack/TestStack.Seleno/pull/168 was merged and I discovered last week when working on another contribution.

I discussed the problem with @robdmoore before starting work on this issue. 

In this PR:
- I have changed the `ScreenshotTest` and `DomCaptureTest` acceptance tests to expose the issue.
- Changed the Autofac registration of the interceptors so that we create an Interceptor pipeline (http://docs.castleproject.org/Tools.DynamicProxy-Introduction.ashx). This was the main cause of the issue as the second `CreateInterfaceProxyWithTarget` registration for the `DomCaptureProxyInterceptor` replaced the `CameraProxyInterceptor`. Needed to use the override and create the proxy with multiple interceptors.
- Finally changed the implementation of the interceptors so that it worked correctly as part of the pipeline (down/up).