wsgi_test.py: add test cases that send content-length explicitly and trigger double content-length header bug

Thanks to Jason Toffaletti for providing the test case.

rename Greenlet.schedule_switch and Greenlet.schedule_switch_later to Greenlet.start and Greenlet.start_later

this should help with CentOS 5.3. Thanks to Jason Toffaletti.

--HG--
rename : examples/connect.py => examples/concurrent_download.py

- rename 'killall' and 'joinall' to 'kill' and 'join';
- add a few methods from multiprocessing's Pool: apply, apply_async, map, map_async, imap, imap_unordered
- remove old execute and execute_async