Documents GET /message/MESSAGE_ID and POST /user/info/
Improves examples/web_viewer to support editing, replying, starring, pinning

Starring and pinning aren't actually implemented in wrapper yet, so
attempts to use them will crash.
Automatically looks up missing attributes of Messages when they're accessed.

Adds a test which looks up messages by ID, without logging in.

Adds methods to scrape transcript, and to scrape markdown source of
messages.

Renamed test_live_messages -> test_wrapper.

Upgrades to BeautifulSoup 4, for use of the .select() method.

ref #51