Redefining bases structure and modules useful, sigproc, features and main.

Signed-off-by: Eduardo Tenorio <embatbr@gmail.com>
Changing 'corpus' and 'corpuses' (wrong plural) by 'dataset' and 'datasets'.

Signed-off-by: Eduardo Tenorio <embatbr@gmail.com>
Adding option to normalize MFCCs by mean at module 'features'.

Signed-off-by: Eduardo Tenorio <embatbr@gmail.com>
Docs and improvement of function 'read_features' in module 'corpus'.

Signed-off-by: Eduardo Tenorio <embatbr@gmail.com>