Fixes level bugs, analytics timers.

- Fixes tall walls; they weren't in the Wall layer.
- Fixes a map bug in level 2 where empty spaces should've been
    invisible walls. Waves were propagating to stray pillars
    across the map.
- Changes analytics timers to measure time of interaction, not
    time that the game is open. Timers are paused after deaths and
    level loads, and resumed on the pillar click.
Fixes links, adds analytics.

- Fixes Linux download link; forgot the .zip! =(
- Adds analytics to download links and author links.