Allow user to specify actual command in command dict

Fixes #16

With this commit, a user can run a command like so:

```
let commandDictionary = [MongoDBCommandDictKey:["distinct": "trials"], "key": "metadata.location"]
var error: NSError?        
let result = myConnection.runCommandWithDictionary(commandDictionary, onDatabaseName: databaseName, error: &error)
```

The command (here, `distinct`) is encoded first, as MongoDB expects, and all other keys are encoded as usual.