Vad2kaldi

The commits implement:
- scripts for alignment of audio using Kaldi (creating the mlf file for utils/vad scripts)
- minor modifications for utils/vad scripts like normalising the transcriptions in mlf file
  and skipping the silence utterances and the missing silence.mlf 
  Filip can check it at https://github.com/UFAL-DSG/alex/compare/vad2kaldi?expand=1#diff-1f5f272f5f02e35fe84ad62bf1645f91R396 
- basic DNN training for Kaldi

TODOS (for another feature branch):
- use Kaldi features instead of the implemented ones in Python so it can be reused for ASR
- ?refactoring the VAD code?