Consistently handle emails

Whether HTML ticket thread is enabled or not, consistently handle the
processing of the email bodies
Provide '--' message for empty messages

Provide fallback html2text mechanism if PHP xml extension is not available
Better conversion of html to text if no xml ext

If the xml extension for php is not loaded, this patch will do a better job
of honoring line break hints in the html.
Merge pull request #29 from greezybacon/issue/consistent-text-handling

Consistently handle emails

Reviewed-By: Peter Rotich <peter@osticket.com>