Change message header to div instead of table

Hi,

Here is a little pull request to change the headers-table for the message view by div and span.
I added proper id to the divs so you can hide them with css if you want.

Of course themes need would need to be updated, but few lines of css.
While doing this I saw some html generated code containing style attribute, wouldn't it be better to assign class and do the styling only in CSS ?
example :  $html = html::div(array('id' => "all-headers", 'class' => "all", 'style' => 'display:none'), html::div(array('id' => 'headers-source'), ''));

Thanks
Alo