Switch to using fork of urwid as dependency

The previous workaround for getting a git version of urwid wasn't
working properly with pip, so I uploaded a fork of urwid to PyPi
instead.
Version 0.2.1
Use cookie to get persistent connection

Fixes #15.

Google uses a cookie called "S" to implement persistence through their
load balancer. This cookie is set when creating a session or
long-polling request. By sending this cookie with future requests, we
can keep a persistent session, even when opening new connections.
Update gitignore
Allow closing conversation tabs

Default keybinding is ctrl+w.
Add initial installation and user guide docs

The current version is now set in hangups/version.py so both setup.py
and Sphinx have access to it.
Link to docs in the readme
Add Conversation.unread_events
Show unread number in conversation picker
Fix handling of latest_read_timestamp=0
ui: Use the server's count of unread messages

Instead of tracking the number of unread messages locally, use
last_read_timestamp so it's synchronized across devices.
Recognize URLs when sending messages

Fixes #26

Thanks to @mtomwing for the elegant segmentation code.
Remove forgotten warning log