Optional serialization via gedcomx-php

[justincy]

This PR adds support for optional serialization and deserialization via gedcomx-php.

Enabled by setting the objects option to true.

$fs = new FamilySearch([
    'objects' => true
]);

The gedcomx-php library must be installed and included separately.

When enabled, responses will have a gedcomx property which is a class from gedcomx-php representing the response data.

When enabled, gedcomx-php objects can also be set as the body of requests.

TODO:

• [x] Documentation
• [ ] Update gedcomx-php to support error response bodies