Icons for different kind of gatherings (issue #2)

I have extracted the list of gathering kinds into an object.

At the moment, the app assumes that all gathering kinds have a corresponding icon, there should probably be an acceptance test for that which I might add later on.

Some of the icons we use are not public domain and we must credit their authors.  
I've added links to the icons page into an "About" page which is not linked anywhere yet because I did not want to interfere with the layout. /cc @hannesfostie ;)