metadataDocumentFactory actor moved to application level
removed unnecessary fields
unused import removed
xml: UpdateString implemented
GetContent implemented
xml processing refactored to synchronous operation

the asynchronous approach proved to be too clumbersome to implement the
required metadata processing
MetadataGenerator actor added
bootstrapping of h2 en jetty moved into separate class
GetContent implemented
GetDatasetMetadata implemented
FutureUtils.map implemented
map, flatMap added

almost identical to the map and flatMap methods of Future except it does
not require an executionContext.
MetadataGenerator improved
added service linkage methods; added helper methods for adding and
removing nodes
MetadataGenerator activated
various small MetadataGenerator bugfixes
added dataset and service metadata for testing
MetadataGenerator.processDataset implemented
uuid column added to publisher.dataset table
fake uuid generation idea added as comment