logo fixups

[toby]

With the text logo I'd like the tag line to be under the site name so some css like

float:left;clear:left;font-size:13px;line-height:0.1;

Also the site name can be larger if no tagline

commit daac54dbd856 I've added a new global g.header_css that sets the hgroup class to

'header-image' 'header-text-logo' 'header-text-logo-tagline'

I think maybe we should also have 'header-image-css' for the equivalent of the class="logo" in the link to allow/stop the image replacement