Support pre-processing of to-be-served binaries

In order to support preprocessing of to-be-served binaries, we're currently forking the BinariesServet class. We add meta-data to the binary page just before it gets inserted into the cache. The use case is to impose a project-specific licensing scheme on binaries.

To be able to get rid of the fork, we locally added a few extension points, such that we can subclass the BinariesServlet to add the project specific logic to it.

Marijan has discussed this with Ard in the past, and here's the issue for it. I'm attaching a diff with our changes, relative to the site-toolkit trunk on 12/01/2015 (POM version 2.29.00-SNAPSHOT).