support earlier draft specs

[karenetheridge]

With the about-to-land refactoring of the keyword implementations into separate vocabulary classes, it's becoming more doable to support historical draft specs as well (although we will likely impose the stricter definition of output formats onto earlier implementations also).

• requires changing the namespaces of the various classes
• some extra code will be needed in the "core" implementation as well (e.g. URI resolution semantics have changed over time).

This gets us closer to being backwards-compatible with JSON::Schema and being able to borg that namespace.

[karenetheridge]

draft7 is now supported, as of version 0.513. There are no plans to add support for drafts 4 and 6 at this time (although the code already exists in a side branch and it wouldn't be super hard to add it if needed).