Convert to .executemany() where appropriate

[Chris Nelson]

As noted in ​http://trac-hacks.org/ticket/11131#comment:3 and ​http://www.python.org/dev/peps/pep-0249/#executemany, .executemany() is preferred over passing a list of tuples. This can clean up a number of database accesses, potentially making them more portable and readable.