Units uuids extracted to the 'uuids' gem [passes]

== Addons

* 'uuids' runtime dependency.
* Okei::Units includes Uuids::Base.

== Changes

* Uuids declared by the Uuids::Base.has_uuids class helper.
* In a request body "uuids" array added instead of "uuid" string.
* migrations' timestamps updated so the 'uuids' migrations to precede the module's ones.