docs: add a docstring to `Dotkeys` class
iproute: support callbacks on a message arrival

Bug-Url: https://github.com/svinota/pyroute2/issues/14
Merge pull request #17 from chantra/pyroute2_netem

merge netem code from @chantra
netem: pep8 fix
Merge pull request #1 from svinota/netem

master rebase and pep8 fix for netem branch