PDF builder support

Created originally on Bitbucket by Anonymous

The 'roottarget' / 'logotarget' options work fine for the html builder, but since sphinx's pdf builder doesn't know about them, it doesn't order pages as one would expect (and has some other minor issues when used with Cloud).

Since Cloud is primarily an html theme, getting this working isn't a high priority, but it would still be nice to be able to build PDFs (e.g. on ReadTheDocs). This might be possible with just some conf tweaks, but will probably require a custom extension to register some hooks.