Added compatibility for Mongo replica sets.

Replica sets can now be included by adding an array of hosts to config, rather than a single host.  For example:
```
"hosts": [
  { "host": "localhost",
    "port": 27017 },
  { "host": "localhost",
    "port": 27018 },
  { "host": "localhost",
    "port": 27019 },
]
```