Additional options for fog object.

Applies patch based on https://github.com/rumblelabs/asset_sync/pull/264:

From 6564371c5aa541fd6a9313c2978d3b311a7bfd65 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?R=C3=A9mi=20Pr=C3=A9vost?= <remi@exomel.com>
Date: Sat, 3 May 2014 07:49:20 -0400
Subject: [PATCH] Add `fog_additional_options` to AssetSync::Config