Sms support

This large PR adds support for SMS submissions in formhub.
Please refer to the emails to get a sense of what it does.

This PR includes:
* 4 Generic JSON endpoints:
 * single submission with form `id_string`.
 * single submission without `id_string`.
 * multiple submissions with form `id_string`.
 * multiple submissions without `id_string`.
* HTTP-gateway providers:
 * Ushaidi's SMS Sync (App for Android)
 * Telerivet.com
 * Twilio.com
 * TextIt.in (responses not suported so far)
* Creating a submission (`Instance`) from text message.
* Checking the compatibility of a form with SMS behavior (either `error` – incompatible–, `warning` or `success`).
* UI to enable/disable SMS Support.
* UI auto-generated documentation on how to fill Text SMS for a form.
* UI on how to use SMS (URLs of endpoints.)
* A few tests to make sure basic functionality are OK.

Additionnaly, the [Syntax Page](https://formhub.org/syntax/#9-sms-support) has been improved to include SMS features.

There is room for minor improvements which I'll work on but it can be merge now.