Makes the API for creating more than one instance more cohesive

This pull request deals with the features discussed at the issue #57. It has 3 main areas:

* Unifies the use of the `_quantity` parameter through the whole API;
* Treats the methods like `prepare_one` or `make_many` as deprecated;
* Updates the docs;

@vandersonmota I'd advise you to check the diff commit by commit before because I took great care in making it history comprehensive to help your job at merging this big feature.