see https://github.com/SiteSupport/gevent/issues/181 for a discussion

it's not always correct. for example, Popen immediatelly execs another process,
so no Python objects are actually leaked.

this is issue https://github.com/SiteSupport/gevent/issues/116

the mimetools Message class isn't of much help here, but replacing it
probably will do more harm than good. so, we parse multiline headers
on our own.

The
,----
| if 'bytearray' in __builtins__.__dict__:
`----
fails when running under py.test, so I replaced that too.

make sure we deliver exception to MAIN greenlet even if loop.run_callback fails

- update test__socket_close.py

previously it was added to the queue of callbacks.
Update test__systemerror.py

it seems to support them since 0.3.1

- added rawlink() method to Popen
- added linkproxy helper class to gevent/hub.py
- modified examples/processes.py to use gevent.wait()

- rename os.threadpool_read/write -> os.tp_read/write
- replace os.posix_read/write with os.nb_read/write must be only used on non-blocking file descriptors
- add os.make_nonblocking function to make fd non-blocking
- ignore test_thread.TestForkInThread.test_forkinthread

now run_callback is based on a list and a single prepare watcher

- remove loop.callback
- update sleep(0) to use run_callback (and to truly switch to other greenlets)
- updated related tests

- remove test_wait_write_invalid_callback

greenlet does it since 0.3.2 and does it better (stack trace is also restored)

- gevent._util.set_exc_info is removed
- update greentest/test__all__.py
- increase number of tries in debug mode

- test__backdoor.py: do not use create_connection
  it uses getaddrinfo and that may trigger unrelated issues
- test___monkey_patching.py: set timeout to 60
- test__socket_dns.py: wait for the thread pool

Thanks to Wil Tan (#169).

- add gevent.threading monkey-helper module.
- add test__threading.py which also checks that patch_all() does not init the hub.
- update test__all__.py w.r.t gevent.threading
- patched_tests_setup.py: ignore tests in test_threading.py that depend on threading._active not being cleaned up for dummy threads
- update test_threading_2.py

because threading has a module-level lock(), it means that get_hub() was called by monkey.patch_all()

this is not good, in case use wants to fork off children, but don't actually use gevent in master or if
the user wants to configure the hub.

Also, PySemaphore is now removed (don't really see the point of maintaing the same code twice).

- test__greenlet.py: add TestJoinAll0