The great schema extraction

This PR extracts all of the schema into a portable class lib and a separate nuget package here
https://github.com/7digital/SevenDigital.Api.Schema
https://www.nuget.org/packages/SevenDigital.Api.Schema/

This repo consumes schema from nuget, and since it's mostly about the attributes, should be fairly insensitive to schema version changes.
The nuspec lists SevenDigital.Api.Schema as a dependency, so it will be installed with te wrapper's nuget package