Fixed error in test puzzle

Fixed an error in the test puzzle `blankPuzzle` in the test module for
rules. trailing commas in JS arrays do not create an additional element,
so 9 element arrays need 9 commas, not 8.
Defined ruleIsNineByNine and helper function

#21: wrote definition for 9x9 rule, as well as helper function
`isSquare()` which returns true if a puzzle passed to it is a square 2-D
array.