Article slugs are here

- Browse articles via ids or slugs
- If a slug is present and the id is used to lookup an article, redirect
  to the new slugged url
A little less :magic: in the `redirect_to_slug` method
Add tests for missing ids / slugs
Tests for editing an article
Merge pull request #155 from elm-city-craftworks/slugs

Article slugs
Remove unnecessary article creation in test setup
Resolve merge conflicts
Add testing for archives page