Add runHydra shell

runHyda automatically starts hydra and postgres:

```
$ nix-shell -A runHydra
```

The shell receives hydra from the working copy as buildInput.
Running hydra, queue-runner, evaluator and postgres is managed
by foreman (https://github.com/ddollar/foreman) and configured
in `Procfile`.

scripts/start-postgres.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+
+initdb ./.hydra-data/postgres
+exec postgres -D ./.hydra-data/postgres -k $(pwd)/.hydra-data/postgres