Added ability to mark an xform SMS ready and set keyword
Added SMS text to XML Submission Support

* Text parser using pyxform representation stored in xform.json
* Views taking and identity and text to submit text submission returning a single (or list of) dict of status and error message.
Added IntegrityError check on SMS enabling (sms_id_string might colide)
Addec support for SMS-compatibility check on form Upload
Added SMS-checks on clone form too (fixing broken clone by previous commit)
Made all strings i18n ready
Merged with master
Added SMSSync, Telerivet and Twilio providers for SMS
Updated required version of pyxform
Added automatic documentation for SMS filling
Added support for TextIt SMS Gateway (no response support)
Added support for custom sms_response message, Submission ID in response and fixed allow_media
minor code reorg
Changed sms_id to sms_field and sms_option
Fixed unable to disable SMS Support. Added SMS Compatibility info on form page
named urls
Added Usage instructions for all API/Providers
merging with master
Fixed documentation
Fixed required field not mandatory