Make more flexible to customize unserialization of classes #16

We currently have a special condition for DateTime class, but looking at the project forks people are also extending it MongoId and MongoDate. Having an easier way to customize it would be ideal to avoid forks and customization diverging the project.

An idea would be making JsonSerializer receive a list of classname with callable to generate the output. It would be used on the unserialize portion. The current DateTime customization would live in this project since it is a php-core class. People could add it on demand.

Update docs to explain how to implement these customizations too and how to use.