defined URL class in the kanban module
refactored BoardManager to use Url as database URL
refactored the API tests to use the new version of BoardManager
edited typo in BoardTest
renamed persistence to persistency module
BoardManager renamed to FilesystemPersistence
Undo the rename of persistency, the module is persistence again
using a PersistenceManager as an interface to the persistence implementation. used Composition Over Inheritance
Merge branch 'creationOfPersistenceAbstraction' into develop

* creationOfPersistenceAbstraction:
  using a PersistenceManager as an interface to the persistence implementation. used Composition Over Inheritance
  Undo the rename of persistency, the module is persistence again
  BoardManager renamed to FilesystemPersistence
  renamed persistence to persistency module
  edited typo in BoardTest
  refactored the API tests to use the new version of BoardManager
  refactored BoardManager to use Url as database URL
  defined URL class in the kanban module