Link resolver driver improvements (#685)

- added support for datatype additionals in EZB-response for electronic journals
- added support for datatype elements in EZB-response for print journals
- added support for proper status mapping to access classes in VuFind
- extended documentation
- added tests for EZB resolver
- implemented Resolver\Driver\AbstractBase and refactored existing drivers to extend it
- added new required methods to DriverInterface