add and support (core) Plugin method: afterHttpResponse

[Glen De Cauwsemaecker]

There's already the following:

• core plugin: beforeHttpResponse (gets called before an http response has been downloaded, with headers already there)
• client plugin: tabs on resource, gets a limited set of the resources with limited info
• the internal mysql table with all responses (and its requests)

Only the last option can currently be used if you want to track all fetched resources. However, it would be nice to be able to do that from the plugin as well, similar to how we do it for beforeHttpResponse.

Furthermore it would be great if we can add the bytes count of the resources to the returned object there, so we can also keep track of how many bytes were fetched.