Created some useful constants

Also started using them. They may not have the best names or be in the
best place but they are good enough for my purposes right now.
Simplified moveIsValid method
Renamed some constants

It should now be clear which ones are plain arrays and which ones are
associative arrays.
Tweaked findConflicts method

It uses my new constants. The rows, columns, and boxes properties of my
plugin may no longer be needed.
Added getCells method

This method gets all cells whose cell labels match any cell label in an
array passed to it.
Removed rows, columns, and boxes properties
Added getInitialDigitHash method

It returns an object whose keys are the cell labels of the given cells
and whose values are the digits in those cells.
Exposed plugin as a global object

This is only for testing. I may remove it later.
Tweaked handling of empty arrays

I think it is more readable.
Several tweaks to code that generates constants
New capability for populateGrid

It can now populate the grid based on a hash with cell labels and
digits. If no hash is passed, it will populate at random like before.
Removed global variable for plugin

I am will put the testing stuff in my own personal branch. I may come
up with some unit tests or other methodical tests if I have time, but
it is looking doubtful.
Renamed function

For consistent terminology
Added getDigitHash function

It gets all digits, including ones entered by the user.
Improved moveIsValid function

There is now a stand-alone function that does not require a sudoku grid
to be in the DOM. It can be explicitly passed a digit hash.
Added generateRandomDigitHash function

The code for generating a random digit hash is now separate from the
code for populating the grid.