Introducing more Keras related functionality

• Introduced recognizer classes: SceneRecognizer, EventRecognizer
• Introduced Keras callbacks: ProgressLoggerCallback, ProgressPlotterCallback, StopperCallback, StasherCallback
• Added new learner classes: SceneClassifierKerasSequential, EventDetectorKerasSequential
  • Full support for Keras sequential API
  • Epoch-by-epoch processing, with external metric evaluation (using sed_eval)
  • DataProcessor class to embed feature processing and data processing chains
  • Feature generators to allow loading feature data from disk during training procedure
• Added possibility to use constant and simple math equations in the keras based learner parameters