Add support for mongoid dafault timestamp names by flag

For those who use mongo side by sise with ruby code written using mongoid the incompatible var names are problem.
Documented here: http://mongoid.org/en/mongoid/docs/extras.html#timestamps
mongoid uses `created_at` and `updated_at`.
this add on a flag in options, that if `mongoid_comp === true` so this var names are compatible with mongoid defaults.