Integrate a SVN-specific file system.

[Tobias Schlauch]

You have to implement interface datafinder.persistence.data.datastorer.NullDataStorer and datafinder.persistence.metadata.metadatastorer.NullMetadataStorer.

When adding metadata support the properties should be stored using the JSON format. The current implementation uses a specific string-format (datafinder.persistence.metadata.value_mapping.MetadataValue) which was necessary to be able to use the WebDAV-specific property search functionality. This class has to be generalized to support both the textual format and the JSON format.