Removed unnecessary CSS rule

The invalid class is not being used.
Give "cell" class to cells

This is for readability, and maybe avoiding ambiguity if table cells
are ever used for anything else.
Added labels to cells

This uses the notation I have sometimes found: rows are given a letter
from “a” to “i”, columns are given a number from 1 to 9, and cells are
given a letter and a number.
Tweaked populateGrid and moveIsValid methods

I am using the cell labels instead of the row and column indexes.
Keypress changes
Tweaked code used to get rows, columns, and boxes

It is a little bit more complex, but hopefully it should be less
confusing since I am using the row and column indexes I am switching
to, instead of the 0-based indexes I was using before.
Added property to plugin for cells

This may slightly speed up and simplify things.
Added method to get a cell

For readability.
Renamed some variables

For consistency.