Provide different data types for serialization

[dpoposki]

Currently the library supports only JSON serialization/deserialization. Might be useful to allow for configuring the data type, such as serializing to JSON, YAML, XML, etc.

[dmanners]

Again points from Vinai:

Maybe call Sitewards/Setup/Service/Page/Exporter.php and Importer.php something like `FileSystemJsonExporter.php, then other implementations could use other storage facilities or serialization formats.

Since you put an interface on them.

[dpoposki]

@dmanners extracted that point to issue #21