Improve recipes' separation

Recipes are now separated regarding the following concepts: creating
directory structure, cloning/updating repositories, installing the wireshark
and preparing the ping example.
(Motivated by FlowForwarding/LINC-Switch#129).
Improve .gitignore
Enable X11 forwarding and disable starting VM's UI

It's better to forward X11 through ssh to start Wireshark. Anytime the VM's UI
(xfce4) can be enabled by editing the Vagrantfile's `vb.gui` option.
Add ping example

Prepares environment and builds a script to run a basic example of OpenFlow
switch usage. Using the `~/ping_example` script you can easily:
* setup two interfaces that the swich will connect to,
* generate the configuration for the switch,
* run the switch and the controller,
* run the wireshark on the appropriate interfaces,
* send a ping to an interface,
* teardown the environment.