Update REST API version to v2

This is a rough draft for versioning the REST API to v2. I can't say I'm crazy about the approach but it seems correct in that we want to keep the v1 API completely separate. This ensures that any integrations (the WC iOS app, for example) will continue to work as intended and we can be free to add endpoints and break backwards compatibility (as needed) in the v2 endpoints. We can follow this pattern as we version the API in the future by maintaining support for one version back (e.g. when v3 is ready, we'll remove the v1 code and move the v2 code into it's own folder).

If anyone has ideas for a cleaner implementation, please share :)

cc @mikejolley @claudiosmweb @coenjacobs @kloon @thenbrent