Field: Add is_oid parameter.
Field: rename is_oid to oid.
fields: Rename "Nested" to "Embed".

Later on, we'll be able to embed or link to other objects.
tests: Use "Embed" instead of "Nested"
docs: rename "Nested Data" to "Linked Data" ...

... and replace references to fields.Nested with references to
fields.Embed.
Update changelog.
Docstrings: remove/change references to fields.Nested.
Add oid function to schema instances.

(where oid fields are defined)
schema: use utils.make_function, refactor helper funcs ...
schema: rename fuctions.
schema: PEP8
Reorder changelog.
utils: refactor docstrings & comments for suppress.
Add util.reify plus tests. PEP8.
fields: use reify for Embed's schema_inst-attribute
Add rudimentary implementation of Schema.oid()

... there's still lots of cleanup to be done
Add field type "Reference", give it a base class it shares with Embed

This implementation is still lacking, but it's a start.
schema: refactor static methods into functions

... so all helper functions are at the same place (intuitively this
looks better)
schema: rename helper functions, add docstrings
test_fields: parametrize tests for linked object fields