Moved transcription related code outside of Sentence model

This doesn't bring any new feature, it's just a refactoring of the `Sentence` model, which contained a whole bunch of code for the auto-generated transcriptions/transliterations.

I've basically reorganized things by creating an `AutotranscriptableBehavior` and added this behavior to the `Sentence` and `Translation` models.
I copy-pasted all the transcription code from the `Sentence` model into a `Autotranscription` vendor, which is used in the `AutotranscriptableBehavior`.

@jiru and @alanfgh, if you have time could you test this before I merge? It requires to have the various transcription tools installed. Also, don't forget to add in your `config/core.php`:
```
Configure::write('AutoTranscriptions.enabled', true);
```
You want to update the comment above as well.