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?
well, the problem with using MFCCs from KALDI is following, 
for VAD, I use output of mel filter banks now. It was just convenient to use the MFCCFrontEnd class to output them.

For the current VAD, it does not make much sense to use KALDI's MFCCs.