Removed safe parameter due to PyMongo deprecation warning

I get the following warning when using the MongoDB backend:

```
/eggs/pymongo-2.5-py2.7-linux-x86_64.egg/pymongo/common.py:592: DeprecationWarning: The safe parameter is deprecated. Please use write concern options instead.
  "write concern options instead.", DeprecationWarning)
```

Evidently, [PyMongo has deprecated a lot of things](http://emptysquare.net/blog/pymongos-new-default-safe-writes/) in order to set forth a new API to do safe writes by default. Essentially, we need to migrate this backend away from the `pymongo.Connection` class to the `pymongo.MongoClient` class for interfacing with the database.