Added **kwargs to Collection.find method in order to allow MongoEngine t...

Added **kwargs to Collection.find method in order to allow MongoEngine to use MongoMock instead of PyMongo.

This change don't break anything, but allows MongoEngine to use MongoMock instead of PyMongo (MongoEngine also requires minor changes that I'm also pull requesting to their repository).

I'm open to any suggestion to achieve the same result if this change don't seems to you OK.

Thanks for your attention :) .