Modify AbstractTransport class for supporting KeepAlive options

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I'd like to propose a change for the AbstractTranspor class for adding a 
**kwargs parameter.

This parameter could be used to pass specific option for sockets, in particular 
options for defining values for KEEPALIVE settings:
socket.TCP_KEEPIDLE
socket.KEEPINTVL
socket.KEEPCNT

If those keys are found the values are defined.
We could also change the code to verify if a key is passed for the SO_KEEPALIVE 
option for enable/disable it.

The aim of this change is to allow to define specific params for keep_alive 
instead of using default OS values.

Original issue reported on code.google.com by andrea.rosa on 12 Jul 2012 at 3:51