rename `Relation#uniq` to `Relation#distinct`. `#uniq` still works.

The similarity of `Relation#uniq` to `Array#uniq` is confusing. Since our
Relation API is close to SQL terms I renamed `#uniq` to `#distinct`.

There is no deprecation. `#uniq` and `#uniq!` are aliases and will continue
to work. I also updated the documentation to promote the use of `#distinct`.
Deprecate the `:distinct` option for `Relation#count`.

We moved more and more away from passing options to finder / calculation
methods. The `:distinct` option in `#count` was one of the remaining places.
Since we can now combine `Relation#distinct` with `Relation#count` the option
is no longer necessary and can be deprecated.