API change: db2.authors_with_sort_strings

[chaley]

FYI in case any plugin developer uses this API:

The return value of the method authors_with_sort_string in library.database2.py has changed. The method returns the authors for a given book. Formerly it returned a list of tuples containing two values: (author_name, author_sort). It now returns a list of tuples containing three values: (author_id, author_name, author_sort); author_id has been added. Author_id is the id of the author in the authors table. The other two values are unicode strings. Note that this method replaces '|' characters in the author's name with commas.

In addition, a new API has been added: books_for_author( id_, index_is_id=False). Given an author_id, the method returns a list of ids of books referring to that author. It returns [] if no books are found.