function create using write concern

discussion of the problem here: https://github.com/1602/compound/issues/619

I took a look at what has changed in mongodb version 0.9 to the current and I found the problem. 
In the newest version of the mongodb module, it treats the concept of writing concern, which I found here: http://mongodb.github.io/node-mongodb-native/api-generated/collection.html#insert 

In this pull request I treated the setting to write concern