added an appdata file for corebird.

Added an [Appdata](http://people.freedesktop.org/~hughsient/appdata/) file for corebird.

Please check that i have also added the right stuff for cmake (wasnt 100% sure what to do here.)

these files are used by the new [gnome-software](https://git.gnome.org/browse/gnome-software/) application. Note that the screenshot in appdata require an external link, so i am currently hosting them on my webserver, but can move them over to the official corebird site if needed.

Also, appdata requires a scecific aspect ratio for the screenshots, so i couldnt just use the ones already corebird site. :(

The screenshots are:

![corebird](http://ryanlerch.org/corebird/corebird.png)
![corebird](http://ryanlerch.org/corebird/corebird2.png)
![corebird](http://ryanlerch.org/corebird/corebird3.png)