Refactors the PagedEnumerator into the EcwidPagedResponse

This makes most of the Ecwid specific stuff for the PagedEnumerator
and makes it so it can be used easily with other response types.
Adds pry for development
Requires Error instead of Autoloading it
No more method_missing for Entities in favor of Ecwid accessors

method_missing is rarely a good idea... this commit makes it
very clear which properties are available on each of the entities.

This commit also introduces a `#url` method to each of the entities
that can be used for `save` and `destroy!` operations on an Entity.
Allows accessors to be overridden and still retain super
Updates Orders for API v3 and new Entity models