Rewrote gameblock.js with CoffeeScript.

The hardest part was translating this JavaScript:

for (i = start; i < finish; i++, y += y_dir)

I naively had:

for i in [start...finish]
  y += y_dir
  ...logic that uses y...

However, in the original JavaScript, the y increment only
happens after the loop's logic is executed. The fix:

for i in [start...finish]
  ...logic that uses y...
  y += y_dir

I also had to learn how to do the jQuery handler functions. I used
this:

http://stackoverflow.com/questions/10969788/translate-this-jquery-into-coffee-script

for inspiration, but I used a more 'declarative' syntax (naming my
handlers, instead of having them be anonymous).

This changed of course meant changing the index.html to reference this
new file.
Removing gameblock.js.

Could this be the last JavaScript file I ever write? Probably not, but
it's a nice thought!