now supports other operators - multiply, greater, and less than:  .../mathtest/index.html?base=16&min_op1=1&max_op1=16&min_op2=0&max_op2=16&auto_next=0&operator=*
scaling text properly now.
Allow the num_buttons to be even bigger- may want to scale them up if base is less than five.
Scale buttons up even more for small bases
Make button arrangements better for multiples of 4