Socket Factory: Use SOCKOPT_LINGER by default.

By setting ZMQ::SOCKOPT_LINGER to 0, the sockets will be destroyed even
when there are still messages to be sent.

Fixes #15.

Signed-off-by: Jacob Kiers <jacob@alphacomm.nl>
Merge pull request #16 from jacobkiers/fix-socket-factory

Socket Factory: Use SOCKOPT_LINGER by default.