Custom translations for system SMS responses

Similar to custom translations in CommCare apps, this allows for custom translations of system-generated SMS messages, such as responses to invalid choices in an SMS survey.

![translations-1](https://cloud.githubusercontent.com/assets/1028814/2890915/a5d2ae82-d52d-11e3-88ce-6fbc2f495ffb.png)
![translations-2](https://cloud.githubusercontent.com/assets/1028814/2890916/a7e87580-d52d-11e3-9168-e4d78e775b03.png)