Allow use of non-id primary column

Closes #239.
Add specs, fixtures, and model for non-standard primary keys
Don't rework how scope works
Add parallel specs for categories and users [broken]

I don't know why these are broken. Is it because we're using
fixtures? I don't know.

Pretty much all of the specs that are broken for
models/users are also broken in models/categories,
so I expect this is an issue with the testing environment
rather than an issue with the actual code.
Ensure UUID on Users models
Remove duplicate categories_spec
Fix rebuild! to use parent_id instead of id
Change to 'new_parent_id' to be more accurately named
Keep order for rebuild the same (use model primary key)
Order by specified primary key column when validating duplicates
Merge pull request #241 from shekibobo/named-primary-key

Allow use of non-id primary column