Add CanonicalSlugDetailMixin to braces.views.
Add documentation for CanonicalSlugDetailMixin.
Add tests for CanonicalSlugDetailMixin.
Make failing CanonicalSlugDetailView tests pass.
Add @folz to CONTRIBUTORS.txt
Merge pull request #61 from folz/canonical-slug-detail-mixin

Canonical slug detail mixin, with docs and passing tests.
merge with master