Create cool-retro-term.appdata.xml

First attempt in an AppStream metadata file according to http://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html

To do: A "make-release" script that fills in version number, timestamp, and changelog.
Merge pull request #115 from KAMiKAZOW/patch-1

Create cool-retro-term.appdata.xml