Recognize super and sub categories when creating a Category object from json

* Remove catch clauses, throw exceptions in GET methods
* Remove BusinessObjectRessource.class
* Move constants form BusinessObjectRessourceClass to subclasses

* Implement conversion of related BOs from Json
* Implement method BusinessObject.equalsInAllProperties
* Fixed spelling mistake in test resource TestApplication.json

* Replace hard coded relation fields by Map of relations
* Make methods in subclasses generic, move to super class

* Use DatabaseControllerHelper extends DatabaseController for test
cases. This keeps track of all created BOs during the tests, so they can
be deleted by @AfterClass method.
* Refactor the relation storing tests to use a Dataprovider - remove
duplicate code.

* Make DatabaseController and SqlStatements classes non-static
* Rename SqlStatements to SqlStatementFactory
* Make Connection a field of SqlStatementFactory, so there are no longer
new connections for every request