Changed the way the file structure is parsed.

Aegisub, one of the major ASS file editors changed the way it stores it's
own metadata in ASS files. It creates a new section, usually something
like "[Aegisub Project Garbage]", and puts all the Aegisub only data inside
that section, instead of cluttering up "Script Info".

With this patch, python-ass is able to ignore (or handle) unknown sections.