Connection should emit on events

[cmoesel]

According to the API docs for Connection, the connection object should emit a number of events. A few important ones to consider supporting are 'open' and 'error', since some applications may wait for one of these events before proceeding.

var connection = mongoose.createConnection(mongoUrl);
connection.on('open', function() { console.log('OPEN!'); });
connection.on('error', function() { console.log('ERROR!'); });

[mccormicka]

Added events please see readme update