Add support for new read-write API.

[nihiliad]

The fact the new API, unlike the old one, has no version numbers in its URLs, and may just be unversioned, will present a challenge. In case the new API, once it replaces the old one, uses the same version scheme, keep the validation and support for that version scheme. Our local validation of collection/version-number combinations is valuable because the Pure API's error messages are not very descriptive, e.g. "Requested service was not found, message was: HTTP 404 Not Found" Find a way to support the currently unversioned new API alongside the old one.