Expose keeaplive settings

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

Those values can be set passing a dictionray where the key is the name of
the parameter we want to set.
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