Add appdata.xml file

This file can be shipped in Linux packages and allows the Gnome
application browser to show some additional metadata.

This is essentially just the file from #1572, with a typo fix and a comment added pointing to the gnome wiki. Note that the updatecontact address still points to mavit@. I looked at pointing to the development mailing list or something generic like that, but that doesn't allow non-member postings, so I guess mavit can just keep the file up-to-date if needed :-)

The screenshots also still point to mavit's webspace. Not sure what the proper way to do this is. It seems to make sense to include the screenshots in git, but I can't find out if that's possible from the [appdata "specification"](http://people.freedesktop.org/~hughsient/appdata/). Perhaps putting the screenshots on arduino.cc instead is better?