Base template should not enforce header's max-width

[Stanislav N. aka pztrn]

Right now mediagoblin's base theme (aka default theme)) enforces header's maximum width, but this should not happen in my opinion, because can make some troubles to theme designers.

​http://gitorious.org/mediagoblin/mediagoblin/blobs/master/mediagoblin/static/css/base.css#line116

In my opinion, base.css must be included for _really_ base design (e.g. theme skeleton), and all max-* items must be defined in themes (and colors too). So there is a possible solution: make a default theme, that can be specified in "theme" option in mediagoblin{_local}.ini and clean up base.css.