Added serialization support for nested fields using dot notation.

This change is to support PATCH commands containing updates to nested fields in subdocuments, using dot notation as in MongoDB. An example would be a PATCH like this:

```{"data.fan": "5435c2a4d302e926e818b2d5"}```

The idea is to traverse the schemas of each field in the dotted name to find the appropriate field_schema to use for serialization.