Updates to Send to phone feature

These updates should make the send to phone feature a bit more error-proof and user-friendly.

- Restrict feature to US ip addresses only, using the existing Geocoder gem integration (`request.location.try(:country_code) == 'US'`)
- Change phone number mask from Josh Bush's [masked-input-plugin](http://digitalbush.com/projects/masked-input-plugin) to Igor Escobar's [jquery-mask-plugin](http://igorescobar.github.io/jQuery-Mask-Plugin/)
- Require complete US phone number before submission