Timeout: make __str__ return empty string, so that tracebacks are not cluttered with irrelevant information

greenlet.py: fix bug introduced in 117:ba38996ed7de: spawn_later ignored seconds parameter and launched everythin immediatelly

rename proc.RunningProcSet to ProcSet; add kill() and killall() methods that take care of killing a proc more than once

- RunningProcSet is still available as an alias, although it's deprecated

- avoid wrapper function if no kwargs
- use free function instead of closed one otherwise

- remove TimeoutError
- Timeout and SilentException derive from BaseException now
  (if it's 2.4, BaseException is defined)
- with_timeout function is simplified a bit
- select() bug fixed: it could interfere with outside timeouts

fd is available as event.fd