Identicons

Closes #120. Part of #82.

I pulled in the identicon code because there is no published library, and also because when I used it for Bote I ended up modifying the code somewhat for my purposes; I expect the same to happen here.

I am not sure what information should be used to seed the identicon. I originally thought of using the public key, but that seemed to change the identicon every single page load (obviously the representation wasn't consistent). Then I tried using the `ContactId`, which was fine until I got to adding an identicon to `AuthorView`: activities that use it are only passed (via Intent) a name, not anything else. Hence `AuthorView` identicons are currently inconsistent with the contact list and conversations.

See merge request !51