Phonetic transcription and ambiguous words highlight in redstart

This changeset introduces two new features in redstart:
- phonetic transcription visualization
- highlight of problematic words, so that the speaker can read them properly

![redstart_transcription_red_alert](https://f.cloud.github.com/assets/311723/232029/c518ea5c-871f-11e2-901d-4dd11823e87e.png)