Module review

This issue is for reviewing smsgateway module.

This is my second full project in Drupal, another is views base url. The reason behind reviewing this project is that, this project provides custom API and rules events and action. I am not sure whether I am creating the API in proper way.

Things for code review:
- API integration, its scope and usage
- Rules integration, its scope and usage
- and, code review :)

Any review suggestions are welcome, and I hope it will make this project more usable. Thanks!