CanonicalSlugDetailMixin now inherits from `object`.

I don't believe `CanonicalSlugMixin` should inherit from `SingleObjectMixin`.  Doing so will mask any custom `.get_object()` method that the user may be providing in their base view.

The documentation also states that this mixin works with `DetailView`, making the inheritance of `SingleObjectMixin` unnecessary.
Merge pull request #78 from tomchristie/patch-1

CanonicalSlugDetailMixin now inherits from `object`.