Change Ecto.Repo.Backend.validate_entity/1 handle the result of
Ecto.Query.Util.value_to_type/1 correctly when the value is a list.

Wire up Postgres backend to support lists -> ARRAY properly via an Ecto
type defined as the tuple { :list, type }.
Pull request #139 made quoting a requirement.
Handle type mismatch in Util.value_to_type and return error, otherwise
extract inner type and return in the expected tuple format.
Merge pull request #145 from zoips/postgres_arrays

Make :list type translate into Postgres ARRAY