refactored code from issue #45 (added insert method)

I could not figure out how to properly ammend the mentioned pull request so I created my own.
I also added a test and a comment indicating a bug with the write concern. I however did not change that code, before it is certain it is a bug, and I did not want to change too much at once.

This is the original issue:
https://github.com/vert-x/mod-mongo-persistor/pull/45
Slight preference to split out the functionality for generating an id vs storing one.  Can this method be refactored into two separate ones?