Adding the check-for-updates script

First prototype.

Related to: https://github.com/KanoComputing/peldins/issues/647
Adding a status file to /var/cache
Adding the lxpanel plugin

This is a work in progress.
Finalizing the lxpanel plugin
Adding copyright
Merge pull request #6 from KanoComputing/check-for-updates

Check for updates and the lxpanel plugin