Problem updating config data

Hi @alexbilbie.
I've been a fervent user of your previous codeigniter-mongodb-library.
Now I'm migrating to MongoQB as I upgraded php mongo extension version to 1.3.x.
Everything seems work well, but I found something.

When updating `_configData` using `setConfig`, I think the order of parameters passed to `array_merge` is reversed.
I also notices that in default config array, the key named hostname should be changed to dsn.
Please check these out.