Create a board that converts a puzzle string into cells

* Create board that takes a string of cell values
* Create board with empty cells array
* Create "createBoard", which splits the string, makes
  a cell out of each value with the value and position in
  the string, and gets the row, column and box of each cell
Merge pull request #5 from mknudsen01/create-and-populate-board

Create a board that converts a puzzle string into cells