backends: a new package

* Backends were moved to a separate package.
  All backends have their own separate files.

* Amodule package was removed.

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
general: inject x headers

* Adds function which will help to inject
  x headers after request.

* Enhances test suite accordingly.

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>