Tyding up conflicting logic on email alerts

HTML2Text was being called on message, but we're sending messages as HTML.
This was producing nasty emails and sad alertees.

Removed the call to HTML2Text and wrapped functino on text::auto_p() to
produce better looking messages
Updating send class to attach a plain text version to message when $html = true
Cleaning up alerts scheduler
Merge pull request #1249 from ditorelo/943

Tiding up conflicting logic on email alerts (Issue #943)