Adding index to save_date

Fixes #129 as well as a few others that this problem cascaded onto.


Verified that it added the index on a new instance using mongo shell:

```
$ mongo
MongoDB shell version: 2.6.1
connecting to: test
> use dagobah
switched to db dagobah
> db.dagobah_log.getIndexes()
[
	{
		"v" : 1,
		"key" : {
			"_id" : 1
		},
		"name" : "_id_",
		"ns" : "dagobah.dagobah_log"
	},
	{
		"v" : 1,
		"key" : {
			"save_date" : 1
		},
		"name" : "save_date_1",
		"ns" : "dagobah.dagobah_log"
	}
]
>
```


![selfie-0](http://i.imgur.com/cIK1h6p.png)
sorry I havent been submitting my PRs with selfies