Add exportCSV parser
The exportCSV parser takes the array of events from each game's session data
and creates a tab-separated CSV for the user to download. This CSV is named
using the same unique ID as the JSON that the session data was stored in.
Currently, the columns in the CSV are: (from left to right) timestamp, type,
and details.

Related changes:
 - Add unique ID (filename) as an attribute of the exported JSON data.
 - Catch events which are submitted without type/details, and define the
    missing attribute as "no type"/"no details".
Minor changes: Add up-next display, remove piece shadow, etc.
This is a group of minor changes to the Tetris game code, as follows:
 - Add up-next display showing the next piece to fall
 - Remove the drop shadow indicating where a piece will fall
 - Remove pause button and key (game still pauses when unfocused)
 - Change score to be number of lines cleared (used to include hard drop pts)
 - Create landing page with instructions (index.html)
   - Move game to game.html
 - Replace instructions on game page with table of controls
 - Add win condition: clear 20 lines
 - Remove mouse-based control