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
Thanks, @motasem-salem - that's unexpected. With this in mind, I'm even more in favour of refactoring date validations to a one-line like `it { is_expected.to validate_date_of :date_attr }` using custom matchers. What do people think of this approach? What validations can go into the matcher so as to make it meaningful but not to duplicate tests of the validator itself?