Add a serializationFormat parameter to the configuration in order to …

[pfauchere]

…choose between json and xml

Includes:

• feature
• unit test
• documentation
• default value: json

[pfauchere]

@stefanseifert as discussed by email here is the PR for providing an XML serialization format.

[stefanseifert]

thanks a lot - we'll review it! this will solve #4