made HiddenMarkovModelTagger's order stable so unit tests can pass
slight change in naive bays behavior so unit tests make more sense: the 'None' feature is only added when there are actually missing features in some instances
added some comments on `_unique_list`
added a comment on the reason for the `num_samples - count > 0` check
swap matrix columns in comments
Merge pull request #647 from jskda/pull_request.unit_test_fixes

Unit test fixes