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`.

run-hydra.nix

@@ -0,0 +1,16 @@
+{ foreman, mkShell, hydra, netcat, postgresql95 }:
+{ doCheck ? true }:
+mkShell {
+  buildInputs = [
+    foreman (hydra.overrideAttrs (_: { inherit doCheck; })) netcat postgresql95
+  ];
+
+  shellHook = ''
+    export HYDRA_HOME="src/"
+    mkdir -p .hydra-data
+    export HYDRA_DATA="$(pwd)/.hydra-data"
+    export HYDRA_DBI='dbi:Pg:dbname=hydra;host=localhost;'
+
+    exec foreman start
+  '';
+}