Implement prototype urwid UI

Supports picking a conversation, and reading/sending messages in a
conversation with a nice scrolling UI.
Refactor code into ConversationWidget
Refactor code into ConversationPickerWidget
Merge branch 'urwid'
Look up users not in contacts

Since you can be in conversations with users who are not in your
contacts, the initial list of contacts may not include everyone needed
to create the list of conversation names.

fix #4