Remove commented out line in SMSSync error template
Add missing header to plugin PHP files
Add email provider + incoming/outgoing tasks

* Add incoming task to regularly fetch incoming messages
* Add outgoing task to regularly push outgoing messages
* Add fetch() to DataProvider - implements fetching latest messages
  for each provider
* Add process_pending_messages() to DataProvider - processes pending
  messages and sends them
* Add PENDING_POLL status to differentiate a message waiting to be sent
  and message waiting to be picked when the provider polls us.
* Add Email data provider