Refactor model specs

PT https://www.pivotaltracker.com/story/show/76108962

- refactor model specs to uniform format, use shoulda-matchers
- delete empty model spec files
- fix province factory
Realized that the problem lay elsewhere but forgot to restore your version. Fixed in new commit. (The only description of your implementation I found was this http://www.pmamediagroup.com/2009/05/smarter-sequencing-in-factory-girl/ but nothing in the official docs. Am I not looking well enough?)