Replicate init breaks past second node

This should hopefully address #63 and #58.

I was seeing something similar to #63 in my environment. When my 3rd+ node would come up I'd get an error that said "one of the first 2 servers already initiated", which wasn't handled and adding the new node would fail. So, this addresses that.

Also #58 which seems to just be Mongo::ReplSetConnection.new was expecting something different than it was getting. Not sure if this is an upstream change in the Mongo driver.

And now, I can happily launch 3+ instances and have them join the replicaset.  

mongodb-10gen -> 2.2.3
mongo gem -> 1.8.2