Fixes #5079: Add API to manage Techniques

http://www.rudder-project.org/redmine/issues/5079
Sure, it may be common, but it still deserves a comment.

As for where to explain, please just save everyone time and put it in the file. I don't need to read this twice, with different wording, etc. If I can't understand it from the comment you write in the file, then that comment is not good enough, so it's a better test to just put it in that explaining it here (where the future code readers will *not* come)