Fix #95: Add AppData file for package managers

This is a file that is can be used by package managers to present information on a package. See http://people.freedesktop.org/~hughsient/appdata/ for more info on the AppData spec.

One problem is that we cannot really make a screenshot for Redshift that complies with guidelines given in the specification. This is because the specification asks for a screenshot of the application window (and nothing else) and also asks for a screenshot of a certain size, larger than the information window in redshift-gtk. We could solve this by disregarding the spec on this point and choose a screenshot that shows the information window and parts of the desktop. We could possibly try to show the redshift effect using a modified screenshot like the one at http://jonls.dk/redshift/ .

- [x] Add screenshot
- [x] Where should screenshots be hosted?

I'm hosting the screenshot at http://jonls.dk/wp-content/uploads/screenshot1.png for now.