Enable find(..) and findOne(..) to work with setObjectClass(..).

This pull request will enable `find(..)` and `findOne(..)` to work with object classes. This is a followup to https://github.com/fakemongo/fongo/pull/48 which added the base functionality and enabled `findAndModify(..)`.

* `find(..)` to replace all objects with the desired object class
* `findOne(..)` to replace the result object with the desired object class
* unit test for both new methods

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@twillouer please review, merge and delete the branch.