Delay the creation of the connection

By delaying the creation of the client object, the application can start
when the mongodb server is down, making apps easier to manage and keep
running.

Now I don't know if what I did is pretty, I'm not too versed in flask extensions development, but it does work.

This is specially important when the mongo server is in a separate machine as the application, and you can't ensure it will start first.

If the code isn't acceptable, please think of this as a feature request.