Change folders structure, composer.json and readme

 - Changed folder structure form lib to src
 - Changed package name to simpleweb/silverpopphp (same name of the repo)
 - Removed error on EngagePod class

I only changed from lib/ to src/ because this is the structure of most part of the packages using composer.

After this pull request is merged, this package could be added to packagist and then the repositories could be removed from composer.json

Any issue, please let me know.

Thanks, this is a great library, it helped me a lot :)
Romulo De Lazzari