Support string-references for Nested.

E.g. allow back-references or cyclical references.

We could register schema class names whenever Schema is subclassed, which would allow us to do this:

artist = typesystem.Nested('Artist')

A lazy import string would also work:

artist = typesystem.Nested('myproject.schemas:Artist')

[wbolster]

(yeah agreed, hence the parentheses 😉)

[tomchristie]

See the example in #46 for how we’ll approach it here.

We won’t allow string references unless it’s a field embedded within a specific namespace.

Namespaces end up being simple dict-like interfaces, and will play nicely with JSON Schema ‘definitions’/‘$ref’