Fix SSL shutdown_transport function and celery issue #1414

See issue #15.
Moreover, for celery issue #1414, from the commit message :
"In SSLTransport._read, errno.ENOENT must also be catched when reading
from the SSL socket. Moreover, it now uses an internal read buffer, as
in TCPTransport. This is done so that the both implementation are
coherent, but I don't really see the point as the OS already does
buffering!"