Fixed bug in socket.accept(): for sockets with timeout==0.0 it would return unwrapped _socket.socket instance

monkey: fix patch_socket() to expect that ssl may not be present in socket module; also patch sslerror too

- add sslerror to __all__

- rename GreenSSL to SSLObject
- remove is_secure attribute
- add __getattr__ since base class no longer has it
- clear exc_info before switching
- accept() now uses socket.accept()

- add ssl_listener() function

- store the real socket as '_sock' instead of 'fd'; fd is available as a deprecated alias
- __getattr__ is gone; it screws up subclassing; the delegated methods are implemented directly, similar to how stdlib socket does it
- sendall is fixed to never call _sock.sendall (it used to do it when timeout==0.0) - this won't work for ssl subclasses
  it also does not call time.time() twice in a row anymore

- signal error via select.error exception that is imported from stdlib's select

- cancel() is usually used in "finally" sections and raising anything here is not helpful

GreenletSet: make spawn* function work well with Greenlet subclasses (do not assume what the arguments should look like)