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-evaluator.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+# wait for hydra-server to listen
+while ! nc -z localhost 3000; do sleep 1; done
+
+exec hydra-evaluator

scripts/start-hydra.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+# wait for postgresql to listen
+while ! nc -z localhost 5432; do sleep 1; done
+
+createdb -h $(pwd)/.hydra-data/postgres hydra
+
+hydra-init
+hydra-create-user alice --password foobar --role admin
+
+exec hydra-server

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

scripts/start-queue-runner.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+# wait until hydra is listening on port 3000
+while ! nc -z localhost 3000; do sleep 1; done
+
+hydra-queue-runner