v2.19

• again bumped the .so version, as we break the ABI
• Refactored the Class hierarchy for clearer code
  • introducing an AbstractFeature class as base for all derived Features
• improved exception and error handling, including line numbers in messages
  when possible
• added code to detect mismatch between annotators and processors (were not
  detected until now)
• cleaner en better C++ code, const correctness and such...