Made all functions nothrow

zmq_msg_init_data() takes a callback, which must also be nothrow if the
function itself is nothrow.  Alternatively, zmq_msg_init_data() *and*
the callback could be allowed to throw, but since the function is most
likely not written with this in mind, making everyhing nothrow seems
like the safest choice.
Merge pull request #9 from kyllingstad/nothrow

Made all functions nothrow