Update copyright year

Some of the code was modified in 2013, but the year must be the year
when the code is _released_. It will hopefully be released in 2014 :-)
Move some type definitions from File to Buffer

The plan is to remove GtkSourceFile, and make the FileLoader and
FileSaver public.
Make the FileSaver class public

The FileSaver cannot be used as-is, there are still modifications to do.