Version 1.3.10

Service release. Issues resolved: https://github.com/eclipse/paho.mqtt.c/milestone/17?closed=1

There are no external changes. A significant internal change is to use the poll system call instead of select to monitor sockets. Most people should notice no difference, except that poll does not exist on Windows XP as select does. You can use the USE_SELECT compile definition to switch back to use select for those platforms that don't have poll.