Made many little changes, upgraded Sudoku object functionality.

Replaced all X and Y with R and C, respectively.
Removed obnoxious comment block.
Gave Sudoku puzzles a proper __init__ that is more efficient and
includes custom functionality.
Gave Sudoku puzzles an isvalid() function used to check if the puzzle
can be solved*.
!! Note that 3x3 squares are not yet checked.
Added a solved_puzzle to ease bug testing.