Removed spurious comment
Prevent strange behavior with trackpad

The numbers in the cells should no longer change when the user scrolls
with the trackpad or mouse wheel in Chrome. It makes it harder to
scroll in Safari and Chrome but that should not be a serious problem.
Specified explicit font size for heading
Added paper background

Also made some minor visual changes to make the game look better with
the background.

I got the background from http://subtlepatterns.com/handmade-paper/
Visual changes to header
Removed "padding" around header
Tweak to code that solves puzzles
Made solve method return a digit hash

The values are simply the digits that should go in the cells instead of
arrays consisting of a single digit.
Added applySolution method
Tweaked getValues and findConflicts methods

They avoid using jQuery’s map and filter functions, which need
callbacks that confusingly take their arguments in the opposite order
of the built-in functions and Underscore’s functions.
Added isSudokuDigit function

And used it where appropriate.
Minor tweaks
Various optimizations

To getCell and applySolution methods, and to input event handlers.
Optimizations to populateGrid method
Optimization to generateRandomDigitHash method
Optimized createGrid function
Tweaked solver

When there is more than one possible digit for a cell, I am guessing
digits in random order. This may help to generate random puzzles.
Minor formatting fix
Solver no longer requires a grid in the DOM

It can optionally be passed a digit hash.
Minor bug fix to solve method

It works even if the digit hash is missing some keys.