Enabling sort_attribute

[luord]

In a project I did recently, I needed to change the default sort, I saw that this attribute existed in the Meta class for ModelResource, but wasn't being used.

This PR is an attempt of enabling it for usage.

[luord]

Hadn't noticed the conflicts, updated now with the conflicts fixed.

[luord]

Merged the reversion from the other branch.

Don't know if this will ever be merged but, for what it's worth, I think the sort_attribute should be passed as an argument and not decided exclusively from the meta in each manager.

This way loosens coupling, IMO, and the sort could work for any manager and not just sqlalchemy, plus it's more consistent on what the sort is expected to be. Namely a tuple of the field, its name and reverse.

[coveralls]

Coverage increased (+0.06%) to 89.172% when pulling 22a5d64 on luord:default-sort into 66f11dc on biosustain:master.