Update copyright notices

Several files were missing them, and it needed to be updated everywhere
to include 2013.

Signed-off-by: Kyle Moffett <kyle@moffetthome.net>
Check version dependencies in setup.py

Rather than having a specific version in the ifupdown-ng shebang line,
we should instead simply check the Python version in setup.py.

Signed-off-by: Kyle Moffett <kyle@moffetthome.net>