proposal for common data modell of tag and branch
these references now doesn't hold information about their creation; this
information is stored in a referenced commit
Merge branch 'master' of local repository into common_branch_and_tag_model
created common model for tag and branch

merged also methods to create tags and branches to createReference()

createNewRevision() doesn't need a referenceName anymore since all
methods accept now both either a revisionNumber or a referenceName

modified getNewRevisionNumber in a way that it can detect itself if this
is a new branch

adapted several methods which used all changed methods

@stephanhensel: check if this fits also your needs
refactor createtestdataset
Merge branch 'common_branch_and_tag_model'
added test resources
added test revisiongraph and bugfix test dataset wrong URI
bugfix test dataset
correct and simplify testdata queries
fix getReferenceUri() and added new tests