A number of updates

* Error handling: https://github.com/sysapps/raw-sockets/issues/18
* Permission: https://github.com/sysapps/raw-sockets/issues/21
* Local address/port attributes: https://github.com/sysapps/raw-sockets/issues/24
* Naming of join/leave multicast group methods: https://github.com/sysapps/raw-sockets/issues/27
* Erroneous send steps: https://github.com/sysapps/raw-sockets/issues/30
* Added ref to Github repository in header