Expose keepalive settings

This change allows to set some TCP socket options during the creation of
the transport.
The parameters we expose with this change are:
- TCP_KEEPIDLE
- TCP_KEEPINTVL
- TCP_KEEPCNT

Those values can be set passing a dictionary where the key is tha name of
the parameter and the value is its value. The names of the keys are the
ones reported above.
Merge branch 'set_keepalive_settings' of http://github.com/reclaro/py-amqp into set_keepalive_settings

Conflicts:
	amqp/tests/test_transport.py