Docs update: Added goals of this branch
Upd: GameData state transitions refactor complete

GameData attributes are no longer altered by main in state transitions.
An update method was added to GameData to handle any changes reliant on
state (adding/subtracting scores is the major example).

As a result, GameData.clear_players_answered is now a private method.

Also fixed WAIT_CLUE_READ -> PLAY_CLUE_AUDIO transition to match state
flowchart. If no exists, state falls through PLAY_CLUE_AUDIO to
WAIT_BUZZ_IN.
Upd: Controller state transitions refactor complete

Global UI changes previously handled in main's state transition
functions moved to controller. Fixes DRY violation and de-clutters main.
Upd: main.py transition state refactor

Removed unnecessary event.set_allowed calls. These were to prevent
ring-ins during inappropriate states, but are unneeded because
handle_key_event only allows ring-ins if state is WAIT_BUZZ_IN (as it
should).
Upd: Refactor of state transitions (cont.)

States that immediately transition on the next frame, and have a linear
flow (single next state) now handled in GameState.

Branching states are being left in main because their conditions rely on
UI elements, which the game sub-pakage should not see.
Upd: States refactor; AUDIOEND event added

Event added so bulk of WAIT_CLUE_READ -> PLAY_CLUE_AUDIO transition
handled in controller, and so the state set can occur in the event
handling functions of main rather than the "branching" function where it
did not belong.
Upd: Arguments in main changed for consistency
Upd: Custom events moved to root constants.py

Main exclusively handles events, so having the custom events defined in
UI's constants didn't make much sense. I believe they are better suited
to the root constants.py
Docs: Update to v0.911

I'm calling this v0.911 as no changes noticeable to the end-user have
occurred, so v0.92 doesn't seem justified, but the previously poor state
of the state-transition functions bothered me so much I want this fix
pushed to master immediately.

Also, mistake corrected in README.