Allow changing Riak backend

This PR allows changing the Riak backend.  From the looks of it though, you'll only be able to change it once because the sed expression in `riak.sh` won't find bitcask after the first time a cluster is stood up.  That may be ok.  Fixes #21
This is not setting a default for `DOCKER_RIAK_BACKEND` when `DOCKER_RIAK_BACKEND` is not explicitly set. It should be something like:

```diff
diff --git a/bin/start-cluster.sh b/bin/start-cluster.sh
index fbad4f2..4cdc795 100755
--- a/bin/start-cluster.sh
+++ b/bin/start-cluster.sh
@@ -8,7 +8,7 @@ fi

 CLEAN_DOCKER_HOST=$(echo "${DOCKER_HOST}" | cut -d'/' -f3 | cut -d':' -f1)
 DOCKER_RIAK_CLUSTER_SIZE=${DOCKER_RIAK_CLUSTER_SIZE:-5}
-DOCKER_RIAK_BACKEND=${DOCKER_RIAK_BACKEND:"riak_kv_bitcask_backend"}
+DOCKER_RIAK_BACKEND=${DOCKER_RIAK_BACKEND:-riak_kv_bitcask_backend}

 if docker ps -a | grep "hectcastro/riak" >/dev/null; then
   echo ""
```