Add initial classes and a couple of tests for Board

spec/board_spec.cr

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+require "./spec_helper"
+
+describe Sudoku::Board do
+
+  it "initialized grid with nil values" do
+    board = Sudoku::Board.new(8, 8)
+    board.get(0, 0).should eq nil
+  end
+
+  it "puts numbers correctly" do
+    board = Sudoku::Board.new(8, 8)
+    board.put(1, 0, 0)
+    board.get(0, 0).should eq 1
+  end
+
+end

spec/spec_helper.cr

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+require "spec"
+require "../src/sudoku.cr"

spec/sudoku_spec.cr

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+require "./spec_helper"
+
+describe Sudoku do
+
+end