Added support for SMS fields

@ukanga this is a minor change to the code since it only adds a few fields to check on and add. 

Those fields are:
`sms_id` in question list is the special id of the group.
`sms_id` in the choices WD to shortcut values (`name=1`, `sms_id=y`, `label=Yes`)
`sms_id` in the settings WS is the SMS ID/shortcut of the form
`sms_separator` is the character triggering begining of group ("+" in CC+)
`sms_allow_media` true/false whether to allow binary data (as base64)
`sms_date_format` in settings: strptime-compatible format of date input
`sms_datetime_format` in settings: strptime-compatible format of datetime input

For a more visual example, please take a look at pyxform/tests/example_xls/sms_info.xls

I think this is minor and doesn't break anything but: 
* How appropriate are those field names?
* Could'nt find proper documentation so it will probably be only documented formhub-side.