Do not install Xfce4 by default

Installation of Xfce4 involves installing a lot of dependencies. This results
in quite a long daly while provisioning the VM. In addition the GUI
is not a must as the machine is fully functional without it.
Extract Wireshark with OF dissector installation to a separate cookbook

Recipes for installing Wireshark and OF dissector are now in separate cookbook
so that they can be easily reused and better maintained.
Improve README

Typos fixes, information update and layout improvements.
Merge pull request #1 from mentels/dev

Make VM GUI optional, extract wireshark to separate cookbook, improve README