nicer error messages for asserts in wait_reader() and wait_writer()

greenlet.py: rearrange classes - put user visible stuff at the top and internal classes at the bottom

more responsive polling in kill[all](block=True); add join/joinall functions that just wait for a greenlet[s]

add test__queue.py: test for gevent.queue; based on the old tests for coros.Channel and coros.Queue plus a few new ones

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