Investigate different 3rd party voice APIs

[msmollin]

The general opinion for iOS is that AVSpeechSynthesizer is mediocre for English, and poor for anything else. Mapbox had a whole blog post about the topic:
https://blog.mapbox.com/a-complete-turn-by-turn-navigation-ui-in-your-ios-application-edec94640ce7

We should look into them as well as Watson (https://console.bluemix.net/catalog/services/text-to-speech/) for making a better experience internationally as well as in the US.