[CHANGE] Service is not Dependable by default

To make the Service Dependable, extend it explicitly

  class SomeService < Hexx::Service
    extend Service::Dependable

    depends_on :another_service, default: AnotherService
    # ...
  end

This allows not to overcomplicate services without necessity.

== Changes (backward-incompatible)

* Hexx::Service - Hexx::Dependable excluded
[BUG FIX] Helpers::Dependency error message
[REFACTORING] Hexx::Service
[DOCS] Minor documentation updates
[CHANGE] Hexx::Helpers -> Hexx::Creators

The Helpers module is reserved for extraction helper methods
from the Hexx::Service class.
[REFACTORING] Helpers module added

The submodules contains helpers for the Service

== Addons

* Hexx::Helpers
* Hexx::Helpers::Messages
* Hexx::Helpers::Parameters
* Hexx::Helpers::Validations
* Hexx::Helpers::Exceptions
[CHANGE] Service::Invalid -> ServiceInvalid, Service::Message -> Message

Service class refactored. Invalid and Message nested classes removed
from the Service class to Hexx module.

== Changes (backward incompatible)

* Hexx::Service::Invalid -> Hexx::ServiceInvalid
* Hexx::Service::Message -> Hexx::Message
* Hexx::Service#with_callback became private

== Refactoring

Helper methods extracted from Service to a corresponding Helpers.
[VERSION] v7.0.0

== Changes (backward incompatible)

* Hexx::Service::Invalid -> Hexx::ServiceInvalid
* Hexx::Service::Message -> Hexx::Message
* Hexx::Service#with_callback became private
* Hexx::Helpers -> Hexx::Creators
  (changes may be needed in hexx-active_record)
* Hexx::Service doesn't extend Hexx::Dependable by default