Implemented backing mongo storage for modules (#64)

* mongo storage initial impl

* added tests

* mongo tests passing

pkg/storage/mongo/saver.go

@@ -0,0 +1,16 @@
+package mongo
+
+import "github.com/gomods/athens/pkg/storage"
+
+func (s *MongoModuleStore) Save(baseURL, module, version string, mod, zip []byte) error {
+	m := &storage.Module{
+		BaseURL: baseURL,
+		Module:  module,
+		Version: version,
+		Mod:     mod,
+		Zip:     zip,
+	}
+
+	c := s.s.DB(s.d).C(s.c)
+	return c.Insert(m)
+}