notary: fix build

The package is broken on master for some time now:
https://hydra.nixos.org/job/nixos/trunk-combined/nixpkgs.notary.x86_64-linux/all

The main reason for the breackage is that the `Makefile` script attempts
to retrieve the latest git commit by using `git rev-parse` which breaks
as `git` is not in the build environment. This could be fixed by using
`?=` rather than `:=` for the `GITCOMMIT` variable in the `make` script
to easily override `GITCOMMIT` in the `buildPhase`.

See the Hydra logs for reference:
https://nix-cache.s3.amazonaws.com/log/ib4qp8h4r8d830ra4fah38l7ybb82gp7-notary-0.6.0.drv

Furthermore some refactoring was applied:

* Activated the test suite for `cmd/notary` to confirm the basic
  functionality when building for NixOS.

* Added {pre,post} hooks for `{build,install}Phase`

* Added myself as maintainer to have more people available in case of
  further breakage.