Add support for nested schemas in parameter tables

If the field defines a schema, add any fields from the nested schema
prefixed by the field name.

If the field defines a key schema, add any fields from the nested
schema prefixed by the field name and a * to denote the wildcard.

See also: https://github.com/nicolaiarocci/cerberus/pull/36

This depends on #8 to be merged first.