Add odb and reference wrappers

lib/geef/odb.ex

@@ -0,0 +1,7 @@
+defmodule Geef.Odb do
+  def exists?(pid, id) do
+    :geef_odb.exists(pid, id)
+  end
+
+  def stop(pid), do: :geef_odb.stop(pid)
+end

lib/geef/reference.ex

@@ -0,0 +1,24 @@
+defmodule Geef.Reference do
+  alias Geef.Repository
+
+  def lookup(repo, name), do: :geef_ref.lookup(repo, name)
+  def lookup!(repo, name) do
+    case lookup(repo, name) do
+      {:ok, ref} ->
+        ref
+      {:error, err} ->
+        raise err
+    end
+  end
+
+  def resolve(ref), do: :geef_ref.resolve(ref)
+  def resolve!(ref) do
+    case :geef_ref.resolve(ref) do
+      {:ok, resolved} ->
+        resolved
+      {:error, err} ->
+        raise err
+    end
+  end
+
+end