Feature/refresh entire model

A couple of options that we found handy when working with postgres. Bookshelf by default just does `returning id` to run the same codepath as mysql, I'm guessing? 

But postgres supports `returning *`, and it's often very useful to refresh the entire model on updates and inserts. 

Similarly, you can't override the `returning` option set in `sync.js`, if you need to do any other transformations on the row, because it sets the `returning` option again just before running the query. So the second option is there to avoid setting that, on the assumption that the user will specify the appropriate `returning` in the `fetching` and `saving` hooks. 

I get the sense that these two options are _not at all_ the Bookshelf Way of implementing these features, so I'm happy to work with you guys here to make them into something that fits in Bookshelf proper.

(Also, I would appreciate advice on what tests where would be desirable.)

Thanks!