Testing out a new pagination idiom.
Concrete Pagers strategy :wrench:
This passes, but I still want result types...
NextPageURL also returns an error.

This is useful if the JSON parse fails, for example.
Rename PaginatedLinksPage as LinkedPage.

Mainly because it's shorter.
Use the new pagination in identity/3/services.
Repaginate the identity/3/endpoints resource.
Consume the new pagination in openstack.Client.
Add an error return to the EachPage closure.

Because the Extract functions, at the very least, have an error parameter, and it's
kind of a pain to capture it outside the closure.
Bring acceptance tests up to date.
Rename `advance` to `fetchNextPage`.
Rename ConcretePage to LastHTTPResponse.
:fire: the closure arguments.

They're all the same anyway.
Root package tests pass.
Remove the closures from the New.*Pager calls.
Rename collections files to pagination.
Shuffle around source organization.

Utility -> Pages -> Pagers
Parse into an `interface{}` to accommodate arrays.
Merge pull request #200 from smashwilson/pagination-improvements-question-mark

[wip] Alternative pagination idiom