Add support for ReplSet and associated options

Hey again @wzrdtales !

Sorry if the title is unclear. Right now in the mongo driver we have the following:
```
db = config.db || new MongoClient(new Server(host, port));
```

And I'm hoping to change it to something which has support for ReplSet and associated options:
```
db = config.db || new MongoClient(new ReplSet(servers, options));
```

---

I think my general approach would be to try to avoid changing the API and add additional optional options to the `config`. Perhaps something like:

```
var mongoClient;
if (config.replSet) {
var hosts = config.replSet.split(',');
var servers = hosts.map(function(host) {
return new Server(host, port, config.replSetOptions);
});
db = config.db || new MongoClient(new ReplSet(servers, config.replSetOptions));
} else {
db = config.db || new MongoClient(new Server(host, port));
}
```

Before I went ahead and figured the rest out I figured I'd double check with you that this seems reasonable and you'd be amenable to a PR for this. Also please let me know if you have any implementation requests. And, as always, thanks for the work you do :)

---
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