Issue #37: Use netifaces package instead of non-BSD ifaddrs file

I've removed the ifaddrs.py file and the code is now using the python-netifaces package.

The change is straight forward, but i currently do not have a ROS environment to test it properly.

It has a slightly different behavior with link local addresses. It returns them as "<address>:<interface>", while the other implementation returns them as "<address>. I am not sure if this has an impact and it would be great if someone could quickly test it.