Improving Psi's roster

For a while now, Psi users have been requesting several changes and additions to the roster (or ‘contact list’). These requests include grouping contacts into meta-contacts, nested roster groups, and displaying user avatars in the roster. We have been postponing all these changes to the roster as much as possible, because none of us wanted to touch the roster code, for reasons I’ll explain below. This year, Psi is fortunate enough to have Adam Czachorowski (aka Gislan), a student from the Google Summer of Code, to work on roster improvements.