Make theme loading optional

[ThiefMaster]

It might be nice for some people, but when you use tools to compress all used stylesheets into a single file, automatic loading of theme css is extremely annoying. Just allow specifying a NULL theme_url to assume the CSS is already loaded.

[marcaurele]

I agree too. It's easier to include the CSS in the page header if we want one.
More over, the theme configuration doesn't work version 1.0 with a URL to false. It will still try to load the default stylesheet from this wrong url "falsedefault/style.css".