added themes to style html emails

Style applied is taken from the qa-styles.css file of a theme,
where the current site theme is the default.  The content is
wrapped around a div which is given a default margin of 10px
and left text-align.
add support for custom css files

If the user has a custom-styles folder in this plugin's path,
then any .css files in there will be listed as selectable HTML
themes, with the name "Custom - <filename>", and will be inserted
into the email's footer to style it.

The body was also made into standard HTML5 with a DOCTYPE, header,
body, and footer.
Merge pull request #1 from atavakoli/html_themes

Add themes to style html emails; fix