Add embedding field.

Embedding field is used to store an object. This type must be used with a
schema.

See https://github.com/SollmoStudio/beyond/wiki/Beyond-DB-API-Schema for
more information.
Make reference field return ObjectID.

Currently, reference field returns Future of document for convenience.
However, only the object id is needed in some use cases. This patch makes
reference fied return ObjectID instead. When a user needs the corresponding
document, the user must call findOne.
Fix a grammar error.
Remove invalid comments.

bodyAsJson is already implemented.