Add reset script for raspberry pi

[mhellmeier]

Fix #170

This script will ask the user if he or she wants to

• reset the system to factory settings (this will remove the data folder and the my.config.inc.php file)
  or
• only reset the data (this will only remove the data folder)

The code style is oriented on the installation script (https://github.com/andreknieriem/photobooth/blob/master/install-raspbian.sh)

[mhellmeier]

If this PR will be accepted it would be great if someone can mention this sh script in the wiki.

It can be executed with sudo bash reset-raspbian.sh on Raspbian.

[sualko]

It's similar to the reset function in the admin settings, but some people (like me) still like a shell option.