Fixes #5079: Add API to manage Techniques

http://www.rudder-project.org/redmine/issues/5079
I'm really not convinced about this API.

HTTP Headers are not designed to pass data, they are supposed to contain metadata about the HTTP connection (IPs involved, authentication, content negociation, caching, etc). Using them for data seems inappropriate to me.

Also, this seems awkward to use. Sure, most HTTP APIs and clients support passing random headers, but this is "yet another field" to specify.

Why don't we simply use a full URL, like "DELETE /api/techniques/bundle_name"?