Canvas TD - modifications for this site

Original: https://github.com/tddyco/canvas-td
Commit:   3309b741e7bf86cacb7fd3c1938ac20442c3bbbc
Author:   Teddy Cross (MIT, see LICENSE and README.md - unchanged)

Changes:
1. Removed Google Analytics: the ga.js loader in index.html and every _gaq.push()
   call in scripts/ui.js, scripts/utils.js and scripts/game.js.
2. Removed the Google Fonts stylesheet (Droid Sans Mono); index.css now uses the
   system monospace font.
3. Removed the old Twitter high-score feed: the three Twitter score lists on the
   scores page, the unused ui.handletweets() function, and the "tweet yours" link
   on the game-over panel. Local top-5 scores per map (localStorage) are unchanged;
   the local Backtrack column is made taller in index.css to keep the column divider.
4. Start page: the source link now points to the current repository
   (github.com/tddyco/canvas-td) and the link to the author's old site was removed.
5. About page: kept the author's text, but external links were turned into plain
   text and the link to legacy.html (not included here) was removed with its clause.
6. Removed the "a game is currently running" beforeunload prompt so the game frame
   never blocks the host page from navigating.
7. Fit to frame: added scripts/fit.js, which scales the fixed 800x550 game with a CSS
   transform to fit the embed frame; index.css sets body margin to 0 for this. The
   canvas mousemove/click handlers in scripts/ui.js now compute positions with
   getBoundingClientRect() (helpers ui.canvasX / ui.canvasY) so they stay correct
   when scaled.
8. Not shipped: legacy.html, future/, build.sh, TODO.md.
Game rules, balance and graphics are unchanged.