Add a default_handler_class setting for custom 404 pages.

The previous solution for custom 404s (a r'.*' rule at the end of the
handlers list) proved to be undiscoverable for many users, and it
could be nontrivial to put the rule in the right place in the presence
of multiple add_handlers calls and the implicit StaticFileHandler rules.
Fix warnings during final GC of the test suite in python 3.4a1.

3.4 can GC things that were uncollectable before, like abandoned generators.
It also seems generates ResourceWarnings for unclosed files in places
where older versions did not.