Added email sending endpoint to the API. closes #2550
Added email sending endpoint to the API.
closes #2550

- Added new API module named 'mail'
- Added routes for the mail endpoint
- Added 'send a test email' button to the debug settigns page
- Added handler to this button which sends and AJAX request to the mail API endpoint

TryGhost/Ghost-Admin,kevinansfield/Ghost-Admin,acburdine/Ghost-Admin,airycanon/Ghost-Admin,dbalders/Ghost-Admin,acburdine/Ghost-Admin,TryGhost/Ghost-Admin,JohnONolan/Ghost-Admin,kevinansfield/Ghost-Admin,JohnONolan/Ghost-Admin,airycanon/Ghost-Admin,dbalders/Ghost-Admin