Add support for mulitpart mime email to email sending code

Add support for mulitpart mime email to email sending code

The change is backwards compatible so various places that send email
can be upgraded in their own time, or ignore it.
The first user will be the Echo extension but it is likely to be used elsewhere

This requires adding 'pear install Mime_Mail' to the server install
To see a change you need to add $wgAllowHTMLEmail to your LocalSettings.php
By default it is false in DefaultSettings.php as there is no core code that uses it yet

(patch 11 notes fixed)

Change-Id: I2096f50b789ffa0b8f4fa8c32fdaf43e9b0e4661